                     FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

STEFAN SCHNEIDER; ANWAR                     
TANDAR; KOMSU MAMUYA;
MUHAMMAD AIJAZ SATTAR; SANDEEP
HARBANS JAIN; MAHESH
KRISHNAMOORTHY; SARAVANAN
KASTHURI; BOGDAN NEDELESCU,
             Plaintiffs-Appellants,                No. 04-55689
               v.
                                                    D.C. No.
                                                 CV-02-09228-DSF
MICHAEL CHERTOFF,* Secretary of
Homeland Security; EDUARDO                          OPINION
AGUIRRE, JR., Acting Director of
the Bureau of Citizenship and
Immigration Services; BUREAU OF
CITIZENSHIP AND IMMIGRATION
SERVICES,
             Defendants-Appellees.
                                            
         Appeal from the United States District Court
            for the Central District of California
          Dale S. Fischer, District Judge, Presiding

                  Argued and Submitted
           December 5, 2005—Pasadena, California

                         Filed June 7, 2006

        Before: Harry Pregerson, John T. Noonan, and
             Sidney R. Thomas, Circuit Judges.

   *Michael Chertoff is substituted for his predecessor, Thomas J. Ridge,
as Secretary of Homeland Security of the United States, pursuant to Fed.
R. App. P. 43(c)(2).

                                 6179
6180      SCHNEIDER v. CHERTOFF
       Opinion by Judge Pregerson
                   SCHNEIDER v. CHERTOFF                6183


                        COUNSEL

Carl Shusterman, Los Angeles, California, for the plaintiffs.

Joanne S. Osinoff, Asst. U.S. Attorney, Los Angeles, Califor-
nia, for the defendants.

Trina A. Realmuto (briefed), Washington, D.C., for the Amer-
ican Immigration Law Foundation.
6184                    SCHNEIDER v. CHERTOFF
                               OPINION

PREGERSON, Circuit Judge:

   Plaintiffs-Appellants Stefan Schneider, Anwar Tandar,
Komsu Mamuya, and Saravanan Kasthuri (“Immigrant Doc-
tors”) are medical doctors who practice in designated medical
shortage areas. The Immigrant Doctors seek adjustment to
lawful permanent resident (“LPR”) status based on their
employment. The Immigrant Doctors brought an action
against the Secretary of Homeland Security (“Secretary”) in
federal district court for declaratory and injunctive relief,
challenging the validity of several regulations that implement
the Nursing Relief for Disadvantaged Areas Act of 1999
(“Nursing Relief Act”), Pub. L. No. 106-95, 113 Stat. 1312
(Nov. 12, 1999) (codified at 8 U.S.C. § 1153(b)(2)(B)(ii)).
The district court denied the Immigrant Doctors’ motion for
summary judgment and dismissed the action. The Immigrant
Doctors filed this timely appeal. For the reasons set forth
below, we affirm the district court in part and reverse in part.

I.       STATUTORY FRAMEWORK

   At issue in this case is the national interest waiver that pro-
vides an accelerated path to LPR status for immigrant doctors
who practice in medically underserved areas. Absent a
national interest waiver, an immigrant doctor generally fol-
lows a three-step path to LPR status. First, the immigrant doc-
tor obtains a job offer from a United States employer, see 8
U.S.C. § 1153(b)(2), and the employer applies for labor certi-
fication of the job offer from the Department of Labor, see 8
U.S.C. § 1182(a)(5)(A)(i).1 Second, once the Department of
Labor has certified the job offer, the employer files a Form I-
     1
    To obtain labor certification, the employer must show (1) that there are
insufficient U.S. workers to fill the job, and (2) that the immigrant doc-
tor’s employment will not adversely affect the wages and working condi-
tions of U.S. workers. See id.
                     SCHNEIDER v. CHERTOFF                  6185
140, Immigrant Petition for Alien Worker (“immigrant visa
petition”) on the immigrant doctor’s behalf. See 8 U.S.C.
§ 1154(a)(1)(F). Third, the immigrant doctor applies for
adjustment to LPR status based on the approved immigrant
visa petition. See 8 U.S.C. § 1255(a).

   Applicants may avoid the first step of the process by
obtaining a national interest waiver. See 8 U.S.C.
§ 1153(b)(2)(B). Congress first created the national interest
waiver program in the Immigration Act of 1990, Pub. L. No.
101-649, § 121(a), 104 Stat. 4978 (Nov. 29, 1990) (codified
at 8 U.S.C. § 1153(b)(2)(B)(i)), and significantly expanded
the national interest waiver program in 1999 with the Nursing
Relief Act. Although this case focuses on the national interest
waiver program created by the Nursing Relief Act, we first
discuss the 1990 national interest waiver for background pur-
poses.

   The 1990 national interest waiver program gave the Attor-
ney General discretion to waive the job offer requirement for
immigrants whose services to a United States employer would
be “in the national interest.” 8 U.S.C. § 1153(b)(2)(B)(i). That
purely discretionary waiver allowed an alien to skip the first
step, the labor certification, and advance directly to the sec-
ond step, the immigrant visa petition. As a practical matter, an
alien who skips the labor certification process will become an
LPR long before an alien who must comply with the labor
certification requirement. See Liberty Fund, Inc. v. Chao, 394
F. Supp. 2d 105, 110-13 (D.D.C. 2005) (discussing Depart-
ment of Labor “backlog centers” for labor certification appli-
cations and noting that labor certification applications filed by
twelve aliens in that action had been pending without action
for two to four years). The discretionary national interest
waiver continues to exist today in the same form it was cre-
ated in 1990. See 8 U.S.C. § 1153(b)(2)(B)(i).

  In 1999, in response to a nationwide shortage of nurses and
doctors, Congress passed the Nursing Relief Act. The Nursing
6186                     SCHNEIDER v. CHERTOFF
Relief Act created a new non-immigrant visa classification for
nurses who came temporarily to the United States to work.
See 8 U.S.C. § 1101(a)(15)(H)(i)(c). It also created a non-
discretionary national interest waiver of the labor certification
requirement for doctors who agreed to work in federally-
designated health professional shortage areas. See 8 U.S.C.
§ 1153(b)(2)(B)(ii). A doctor would be eligible for the new
national interest waiver if the doctor agreed to work full time
as a physician in an area that had been designated as a health
professional shortage area by the Secretary of Health and
Human Services. See 8 U.S.C. § 1153(b)(2)(B)(ii)(I)(aa).
Congress specified, however, that the doctor could not receive
LPR status “until such time as the alien has worked full time
as a physician for an aggregate of 5 years” in a shortage area,
see 8 U.S.C. § 1153(b)(2)(B)(ii)(II), or an aggregate of 3
years in a shortage area if the doctor had applied for a discre-
tionary national interest waiver before November 1, 1998, see
8 U.S.C. § 1153(b)(2)(B)(ii)(IV).2

II.    IMPLEMENTING REGULATIONS

   On September 6, 2000, the Secretary3 promulgated an
  2
     While the national interest waiver created by the Nursing Relief Act is
operatively identical to the national interest waiver created in 1990, the
waivers differ in two important respects. First, the 1990 national interest
waiver was discretionary, whereas the Nursing Relief Act waiver is man-
datory. Compare 8 U.S.C. § 1153(b)(2)(B)(i) (stating that “the Attorney
General may, when the Attorney General deems it to be in the national
interest, waive” the labor certification requirement) (emphasis added),
with 8 U.S.C. § 1153(b)(2)(B)(ii)(I) (stating that “[t]he Attorney General
shall grant a national interest waiver” of the labor certification require-
ment) (emphasis added). Second, the 1990 national interest waiver simply
waived the labor certification requirement, without imposing any require-
ment that the alien engage in the qualifying employment for a term of
years. See 8 U.S.C. § 1153(b)(2)(B)(i). By contrast, the Nursing Relief Act
restricts national interest waiver recipients from obtaining LPR status until
they have practiced medicine in shortage areas for three or five years. See
8 U.S.C. § 1153(b)(2)(B)(ii)(II), (IV).
   3
     The Attorney General actually promulgated this interim rule. At the
time, the Attorney General was charged with the administration and
                        SCHNEIDER v. CHERTOFF                        6187
interim rule that created two regulations to implement the
Nursing Relief Act.4 See generally National Interest Waivers
for Second Preference Employment-Based Immigrant Physi-
cians Serving in Medically Underserved Areas or at Depart-
ment of Veterans Affairs Facilities, 65 Fed. Reg. 53,889
(Sept. 6, 2000) (codified at 8 C.F.R. §§ 204.12, 245.18).

   The Immigrant Doctors challenge five sections of the regu-
lations in this action. First, they challenge the rule that an
immigrant doctor’s qualifying practice in a shortage area does
not commence until the date of the notice approving the
immigrant visa petition and national interest waiver request.
See 8 C.F.R. § 245.18(e). Second, they challenge the require-
ment that an immigrant doctor who applied for a national
interest waiver before November 1, 1998 practice medicine in
a shortage area for five years if the initial application was
denied before November 12, 1999. See 8 C.F.R.
§ 204.12(d)(4), (6). Third, they challenge the imposition of a

enforcement of the Immigration and Nationality Act (“INA”), and pos-
sessed the regulatory power thereunder. See 8 U.S.C. § 1103(a)(1), (3)
(2000). Congress later transferred that power to the Secretary of Home-
land Security. See Homeland Security Act of 2002, Pub. L. No. 107-296,
§ 1102(2)(A), 116 Stat. 2135 (2002), as amended by Pub. L. No. 108-7,
§ 105(a)(1), 117 Stat. 11, 531. We will refer to the Secretary, instead of
the Attorney General, throughout this opinion.
   4
     We note that the Secretary implemented the interim rule immediately
without prior notice and comment, citing the good cause exception at 5
U.S.C. § 553(b)(B) and (d)(3). See 65 Fed. Reg. 53,889, 53,892 (Sept. 6,
2000). Specifically, the Secretary recognized the immediate public health
need to bring physicians to underserved areas without further regulatory
delay. See id. The Secretary sought public comment and noted that he
would “address those comments prior to the implementation of the final
rule.” Id. Though we do not doubt the necessity of immediate implementa-
tion of the rule, we are concerned that the Secretary has neither addressed
the public comments nor finalized the rule in the more than five years
since its promulgation. Indeed, apart from minor editorial corrections to
the interim rule, see 65 Fed. Reg. 57,943 (Sept. 27, 2000); 65 Fed. Reg.
57,861 (Sept. 26, 2000), the Secretary has left the interim rule untouched.
6188                SCHNEIDER v. CHERTOFF
strictly limited time period during which time the immigrant
doctor must complete the qualifying practice. See 8 C.F.R.
§ 204.12(b), (d)(4); 65 Fed. Reg. 53,889, 53,890 (Sept. 6,
2000). If the doctor fails to complete the qualifying practice
in the allotted time, the doctor’s immigrant visa petition will
be revoked and his application for LPR status will be denied.
See id. Fourth, they challenge the limitation of eligibility for
a national interest waiver to doctors who practice a medical
specialty that falls within the Secretary of Health and Human
Services’ shortage designation for the particular area. See 8
C.F.R. § 204.12(a)(2). Fifth, they challenge the requirement
that an immigrant doctor make multiple submissions to con-
firm compliance with the medical practice requirement before
the doctor can obtain LPR status. See 8 C.F.R. § 245.18(g),
(h).

III.   FACTUAL AND PROCEDURAL HISTORY

   The Immigrant Doctors are all medical doctors licensed to
practice medicine in their respective jurisdictions in the
United States. The doctors seek to adjust to LPR status based
on second preference employment-based immigrant visa peti-
tions filed by their employers in conjunction with national
interest waiver requests. Though the Immigrant Doctors’ orig-
inal complaint named eight doctor plaintiffs when it was filed
in 2002, four doctors have since obtained LPR status, render-
ing their claims moot.5 See Tongatapu Woodcraft Hawaii,
Ltd. v. Feldman, 736 F.2d 1305, 1307-08 (9th Cir. 1984)
(holding moot appeal from revocation of visa petition where
immigrant subsequently obtained LPR status).
  5
   These four are Doctors Muhammad Aijaz Sattar, Sandeep Harbans
Jain, Mahesh Krishnamoorthy, and Bogdan Nedelescu.
                         SCHNEIDER v. CHERTOFF                         6189
  A.       Plaintiffs

      1.    Dr. Stefan Schneider

   Dr. Stefan Schneider is a native and citizen of Germany.
Dr. Schneider came to the United States in April 1992 in law-
ful J-1 non-immigrant status.6 On June 29, 1998, Dr. Schnei-
der obtained a change of non-immigrant status from J-1 to O-
1 status. On September 1, 1998, Dr. Schneider began working
as an infectious disease specialist for Pro Health, Inc. at two
AIDS clinics in California, both of which are in designated
shortage areas. Dr. Schneider continues to work for Pro
Health, Inc.

   On March 6, 2003, Pro Health filed an immigrant visa peti-
tion and national interest waiver request on Dr. Schneider’s
behalf with the Department of Homeland Security (“DHS”).7
On June 26, 2003, the DHS approved the immigrant visa peti-
tion and national interest waiver request. On August 25, 2003,
Dr. Schneider applied for adjustment to LPR status. Dr.
Schneider’s application for adjustment of status is pending.

      2.    Dr. Anwar Tandar

  Dr. Anwar Tandar is a native and citizen of Indonesia. Dr.
Tandar entered the United States in July 1997 in valid non-
immigrant status. Dr. Tandar worked for the Fallon Clinic’s
  6
     Although we need not examine the particulars of the various non-
immigrant visas, the fact that Dr. Schneider initially had J-1 status is sig-
nificant. The Nursing Relief Act explicitly provides that medical practice
engaged in while in J-1 non-immigrant status does not count towards the
aggregate three-year or five-year medical practice requirement. See 8
U.S.C. § 1153(b)(2)(B)(ii)(II), (IV).
   7
     The Immigration and Naturalization Service has been abolished and its
functions transferred to the Department of Homeland Security. See Home-
land Security Act of 2002, Pub. L. No. 107-296, § 471, 116 Stat. 2135,
2205 (2002), 6 U.S.C. § 291. We will refer to the government agency as
the DHS.
6190                     SCHNEIDER v. CHERTOFF
Saint Vincent Hospital in Worcester, Massachusetts, a desig-
nated shortage area, from August 1997 to June 2003. It is
unclear from the record where Dr. Tandar is currently work-
ing.

  On June 1, 1998, the Fallon Clinic filed an immigrant visa
petition and national interest waiver request on Dr. Tandar’s
behalf with the DHS.8 On June 21, 1999, DHS denied both the
immigrant visa petition and the national interest waiver
request, finding that Dr. Tandar had “not established that a
waiver . . . will be in the national interest of the United
States.” On November 17, 2000, DHS denied Dr. Tandar’s
request to reopen or reconsider the denial. On January 12,
2001, the Fallon Clinic filed a new immigrant visa petition
and national interest waiver request on Dr. Tandar’s behalf
under the Nursing Relief Act. The DHS approved the immi-
grant visa petition and national interest waiver request on
September 11, 2001. Dr. Tandar applied for adjustment to
LPR status on November 13, 2001. That application is cur-
rently pending.

      3.   Dr. Komsu Mamuya

   Dr. Komsu Mamuya is a native and citizen of Tanzania. Dr.
Mamuya entered the United States in August 1987 as a stu-
dent in valid F-1 non-immigrant status. Dr. Mamuya changed
from student to temporary worker status in 1996 to work for
Beth Israel Hospital in Boston, Massachusetts. In July 1999,
Dr. Mamuya left Beth Israel and began working for the Fallon
Clinic in a federally-designated shortage area.

   On March 26, 2001, the Fallon Clinic filed an immigrant
visa petition and national interest waiver request on Dr.
Mamuya’s behalf with the DHS. The DHS approved the
  8
   Dr. Tandar’s first application for a national interest waiver was filed
before the Nursing Relief Act, when the Attorney General could, in her
discretion, confer the waiver if she found it to be in the national interest.
                       SCHNEIDER v. CHERTOFF                     6191
immigrant visa petition and national interest waiver request
on September 11, 2001. Dr. Mamuya applied for adjustment
to LPR status on November 28, 2001. That application is cur-
rently pending.

      4.    Dr. Saravanan Kasthuri

   Dr. Saravanan Kasthuri is a native and citizen of India. Dr.
Kasthuri, a radiologist, entered the United States in June 1995
in lawful J-1 non-immigrant status. On January 1, 2001, Dr.
Kasthuri obtained a change of non-immigrant status from J-1
to H-1B status.9 On June 23, 2001, Dr. Kasthuri began work-
ing for Columbia Basin Imaging in Richland, Washington, a
federally-designated shortage area. It appears that Dr.
Kasthuri continues to work there.

   On March 31, 2003, Dr. Kasthuri’s employer filed an
immigrant visa petition and national interest waiver request
on Dr. Kasthuri’s behalf with the DHS. It appears that some-
time between March 31, 2003 and August 9, 2004 the DHS
sent Dr. Kasthuri a request for evidence relating to the immi-
grant visa petition. The DHS contends that Dr. Kasthuri did
not respond to the request, and that it accordingly denied his
applications due to abandonment on August 9, 2004. Dr.
Kasthuri disputes the contention and submits evidence that
suggests that he responded to the request.

  B.       Proceedings Below

  The Immigrant Doctors brought this action for declaratory
and injunctive relief on December 4, 2002 in the United
States District Court for the Central District of California.
Specifically, the Immigrant Doctors sought: (1) a declaration
  9
   As with Dr. Schneider, this is significant because medical practice
engaged in while in J-1 non-immigrant status does not count towards the
aggregate three-year or five-year medical practice requirement. See 8
U.S.C. § 1153(b)(2)(B)(ii)(II), (IV).
6192                 SCHNEIDER v. CHERTOFF
that the five parts of the Secretary’s implementing regulations
discussed in Part II. were inconsistent with, and ultra vires to,
the Nursing Relief Act; and (2) a permanent injunction
enjoining the Secretary from enforcing those portions of the
regulations against them.

  On December 1, 2003, the Immigrant Doctors moved for
summary judgment. On March 26, 2004, the district court
found that the Immigrant Doctors were not entitled to sum-
mary judgment, and dismissed the action, concluding that “the
portions of the Rule objected to by plaintiffs do not imper-
missibly contradict the INA or the Nursing Relief Act.” The
Immigrant Doctors filed this timely appeal.

IV.    ANALYSIS

  An agency’s interpretation or application of a statute is a
question of law that we review de novo. See Halaim v. INS,
358 F.3d 1128, 1131 (9th Cir. 2004); Vernazza v. SEC, 327
F.3d 851, 858 (9th Cir. 2003), amended by 335 F.3d 1096 (9th
Cir. 2003).

   In reviewing an agency’s statutory construction, we must
reject those constructions that are contrary to clear congres-
sional intent or that frustrate the policy that Congress sought
to implement. See Chevron U.S.A., Inc. v. Natural Res. Def.
Council, Inc., 467 U.S. 837, 843 n.9 (1984); see also Natural
Res. Def. Council, Inc. v. EPA, 966 F.2d 1292, 1297 (9th Cir.
1992) (“On questions of statutory construction, courts must
carry out the unambiguously expressed intent of Congress.”).

   In Chevron, 467 U.S. 837, the Supreme Court set forth a
two-part test for judicial review of administrative agency
interpretations of federal law. See id. at 842-43. First, “[i]f the
intent of Congress is clear, that is the end of the matter.” Id.
“Congressional intent may be determined by ‘traditional tools
of statutory construction,’ and if a court using these tools
ascertains that Congress had a clear intent on the question at
                         SCHNEIDER v. CHERTOFF                          6193
issue, that intent must be given effect as law.” Wilderness
Soc’y v. U.S. Fish & Wildlife Serv., 353 F.3d 1051, 1059 (9th
Cir. 2003) (en banc) (citations omitted), amended by 360 F.3d
1374 (9th Cir. 2004).

   Second, if the statute is silent or ambiguous with respect to
the issue at hand, then the reviewing court must defer to the
agency so long as “the agency’s answer is based on a permis-
sible construction of the statute.” Wilderness Soc’y, 353 F.3d
at 1059 (citing Chevron, 467 U.S. at 843). “In such a case an
agency’s interpretation of a statute will be permissible, unless
‘arbitrary, capricious, or manifestly contrary to the statute.’ ”
Id. (quoting Chevron, 467 U.S. at 844).

  A.    Shortage Area Medical Practice that Occurred
        Before the Approval of the Immigrant Visa Petition
        and National Interest Waiver

   [1] The Immigrant Doctors’ first regulatory challenge con-
cerns a doctor’s medical practice in a designated shortage area
that occurred before the approval of the doctor’s immigrant
visa petition and national interest waiver.10 The Nursing
Relief Act states that an immigrant doctor cannot receive LPR
status until the doctor has worked in a designated shortage
area for an aggregate of three or five years.11 See 8 U.S.C.
§ 1153(b)(2)(B)(ii)(II), (IV). The Secretary’s implementing
rule indicates that the medical practice requirement “clock”
begins to run “on the date of the notice approving the [immi-
grant visa petition] and the national interest waiver.” 8 C.F.R.
§ 245.18(e). Thus, under the Secretary’s regulation, any medi-
cal practice in a designated shortage area that occurred before
the approval of the immigrant visa petition and national inter-
est waiver does not count towards the medical practice require-
ment.12 The question then, is whether the regulation that
  10
     We refer to this type of medical practice as “prior medical practice.”
  11
     We refer to this restriction as the “medical practice requirement.”
  12
     It is noteworthy that this portion of the Secretary’s regulation has pro-
found consequences for the Immigrant Doctors in this case. Dr. Schneider
6194                    SCHNEIDER v. CHERTOFF
excludes prior medical practice in designated shortage areas
from counting towards the medical practice requirement is in
conflict with, and is ultra vires to, the Nursing Relief Act.

   Under the first prong of Chevron, 467 U.S. 837, we use tra-
ditional tools of statutory construction to determine whether
Congress expressed a clear intent on the issue in question. Id.
at 842-43. We conclude that Congress clearly intended prior
medical practice to count towards the medical practice
requirement because: (1) the plain language of the statute
makes Congress’s intent clear; (2) a contrary conclusion
would render another section of the Nursing Relief Act mere
surplusage; and (3) Congress expressly excluded medical
practice while in J-1 non-immigrant status from the medical
practice requirement but did not exempt any other qualifying
practice.

   [2] The plain language of the Nursing Relief Act demon-
strates that Congress intended prior medical practice to count
towards the medical practice requirement. Congress required
that an immigrant doctor work full time as a physician in a
qualifying area for “an aggregate” of three or five years
before obtaining LPR status.13 See 8 U.S.C. § 1153(b)(2)(B)

began working in a shortage area on September 1, 1998, but his immigrant
visa petition and national interest waiver were not approved until June 26,
2003, nearly five years later. Those five years of shortage-area practice do
not count towards the practice requirement. Dr. Tandar began working in
a shortage area in August 1997, but his immigrant visa petition and
national interest waiver were not approved until September 11, 2001, over
four years later. Dr. Mamuya began working in a shortage area in July
1999, but the DHS did not approve his immigrant visa petition until Sep-
tember 11, 2001, over two years later. Dr. Kasthuri began working in a
shortage area on June 23, 2001, and still does not have an approved immi-
grant visa petition and national interest waiver. Each of these doctors
would have completed, or nearly completed, the medical practice require-
ment if prior medical practice were considered.
   13
      The five-year medical practice requirement reads in relevant part:
                        SCHNEIDER v. CHERTOFF                          6195
(ii)(II), (IV). We have previously held that “unless otherwise
defined, words will be interpreted as taking their ordinary,
contemporary, common meaning.” United States v. Smith,
155 F.3d 1051, 1057 (9th Cir. 1998) (quoting Perrin v. United
States, 444 U.S. 37, 42 (1979)). Merriam-Webster’s Dictio-
nary defines “aggregate” as “formed by the collection of units
or particles into a body, mass, or amount.” Merriam-
Webster’s Collegiate Dictionary 24 (11th ed. 2005).14 Thus,
applying the definition to this statute, Congress intended that
an immigrant doctor work for a total period of three or five
years in a designated shortage area. Congress intended that

    No permanent resident visa may be issued to an alien physician
    . . . and the Attorney General may not adjust the status of such
    an alien physician from that of a nonimmigrant alien to that of a
    permanent resident alien . . . until such time as the alien has
    worked full time as a physician for an aggregate of 5 years (not
    including the time served in the status of an alien described in
    section 1101(a)(15)(J) of this title), in an area or areas designated
    by the Secretary of Health and Human Services as having a short-
    age of health care professionals . . . .
8 U.S.C. § 1153(b)(2)(B)(ii)(II).
  The three-year medical practice requirement reads in relevant part:
    In the case of a physician for whom an application for a waiver
    was filed under subsection (b)(2)(B) of this section prior to
    November 1, 1998, the Attorney General shall grant a national
    interest waiver pursuant to subsection (b)(2)(B) of this section
    except that the alien is required to have worked full time as a
    physician for an aggregate of 3 years (not including time served
    in the status of an alien described in section 1101(a)(15)(J) of this
    title) before a[n] [immigrant] visa can be issued to the alien . . .
    or the status of the alien is adjusted to permanent resident . . . .
8 U.S.C. § 1153(b)(2)(B)(ii)(IV).
   14
      Black’s Law Dictionary defines “aggregate” as “[f]ormed by combin-
ing into a single whole or total” or “[a]n assemblage of particulars; an
agglomeration.” Black’s Law Dictionary 66 (7th ed. 1999). We note that
the common definition and legal definition of “aggregate” are substan-
tially similar such that reliance on either definition would produce the
same outcome.
6196                 SCHNEIDER v. CHERTOFF
separate, fractured “units or particles” of shortage-area medi-
cal practice could be combined to meet the total three or five-
year requirement. By using the word “aggregate,” Congress
intended to comprehensively include even short periods of
qualifying medical practice to make the requirement flexible.
The statutory language makes clear that Congress contem-
plated counting prior medical practice towards the medical
practice requirement.

   [3] A conclusion to the contrary would render other provi-
sions of the Nursing Relief Act extraneous. “[W]e strive to
avoid constructions that render words meaningless.” United
States v. LSL Biotechnologies, 379 F.3d 672, 679 (9th Cir.
2004) (citation omitted). Congress specified that nothing in
the medical practice requirement should be construed to pre-
vent an immigrant doctor from filing an immigrant visa peti-
tion before the doctor completes the medical practice
requirement. See 8 U.S.C. § 1153(b)(2)(B)(ii)(III) (“Nothing
in this subparagraph may be construed to prevent the filing of
a[n] [immigrant visa] petition . . . by an alien physician . . .
prior to the date by which such alien physician has completed
the [medical practice requirement] . . . .”). The wording of this
subsection demonstrates that Congress contemplated the pos-
sibility that an immigrant doctor might complete the medical
practice requirement before filing the immigrant visa petition;
the Secretary’s regulation does precisely the opposite — it
prevents an immigrant doctor from starting the medical prac-
tice requirement until the immigrant visa petition has been
approved. Indeed, if we were to accept the Secretary’s regula-
tion as proper, subsection (b)(2)(B)(ii)(III) would be surplus-
age.

   Finally, it is a well-established maxim of statutory interpre-
tation that the expression of one thing is the exclusion of
another. See ARC Ecology v. U.S. Dep’t of the Air Force, 411
F.3d 1092, 1099-1100 (9th Cir. 2005) (“[O]missions are the
equivalent of exclusions when a statute affirmatively desig-
nates certain persons, things, or manners of operation.”). Con-
                         SCHNEIDER v. CHERTOFF                         6197
gress expressly exempted from the medical practice
requirement any medical practice performed while the immi-
grant doctor had J-1 non-immigrant status. See 8 U.S.C.
§ 1153(b)(2)(B)(ii)(II), (IV). But that was the only restriction
that Congress imposed on qualifying medical practice. By
exempting J-1 medical practice from the medical practice
requirement while not exempting medical practice engaged in
before approval of the immigrant visa petition, it is clear that
Congress intended that prior medical practice would count
towards the medical practice requirement.

   [4] Congress selected unambiguous language in the Nurs-
ing Relief Act that demonstrates its intent that, with the
exception of practice engaged in while in J-1 non-immigrant
status, all medical practice in designated shortage areas counts
towards the medical practice requirement. By contrast, the
Secretary’s implementing regulation excludes otherwise qual-
ifying medical practice if it occurred before the approval of
the immigrant visa petition. Because the Secretary’s regula-
tion is contrary to Congress’s clear intent, we hold that the
regulation is in conflict with, and is ultra vires to, the Nursing
Relief Act.15
  15
     Although we cannot consider legislative history under the first prong
of Chevron, 467 U.S. at 842-43, we note that the Secretary’s regulation
subverts the very intent of the Nursing Relief Act. Congress created the
Act “to assist the underserved communities of this Nation by providing
adequate health care for their residents.” 144 Cong. Rec. H6931, H6933
(1998). Congress reasoned that, “[b]y allowing alien physicians and the
medical facilities that employ them to avoid the labor certification process,
this provision ensures that residents of areas with a shortage of health care
professionals will have access to quality health care.” 145 Cong. Rec.
H11,321, H11,322 (1999). The Secretary’s regulation contravenes the very
incentive — swift LPR processing — that Congress crafted to attract
immigrant doctors to health professional shortage areas.
6198                    SCHNEIDER v. CHERTOFF
  B.    National Interest Waiver Applications Filed Before
        November 1, 1998

   [5] The Immigrant Doctors’ second regulatory challenge
focuses on the impact that a previously filed national interest
waiver request has on the medical practice requirement. Con-
gress indicated that an immigrant doctor “for whom an appli-
cation for a [national interest] waiver was filed . . . prior to
November 1, 1998” would only be required to work for an
aggregate period of three years in a designated shortage area
before the alien would be eligible for LPR status.16 See 8
U.S.C. § 1153(b)(2)(B)(ii)(IV). The Secretary’s implementing
regulation, however, divides the group of doctors who applied
for a national interest waiver before November 1, 1998 into
two sub-groups: (1) doctors who were denied a national inter-
est waiver before November 12, 1999, see 8 C.F.R.
§ 204.12(d)(6); and (2) doctors whose petitions were still
pending as of November 12, 1999, see 8 C.F.R.
§ 204.12(d)(4). The regulation requires the first group — doc-
tors whose applications were denied before November 12,
1999 — to fulfill the five-year medical practice requirement
instead of the three-year medical practice requirement. See 8
C.F.R. § 204.12(d)(6) (permitting immigrant doctors denied a
national interest waiver before November 12, 1999 to “file a
new [immigrant visa] petition” under the provisions of 8
U.S.C. § 1153(b)(2)(B)(ii), which contains the five-year med-
ical practice requirement).

   Only Dr. Tandar raises this claim. His employer requested
a discretionary national interest waiver on his behalf on June
1, 1998, before Congress enacted the Nursing Relief Act. The
DHS denied Dr. Tandar’s first application on June 21, 1999,
finding that Dr. Tandar had “not established that a waiver . . .
will be in the national interest of the United States.” The Sec-
retary argues that Dr. Tandar must meet the five-year medical
practice requirement. Dr. Tandar argues that, because his
  16
    For the statutory text of 8 U.S.C. § 1153(b)(2)(B)(ii)(IV), see note 13.
                         SCHNEIDER v. CHERTOFF                         6199
employer filed his first application for a national interest
waiver before November 1, 1998, he should only have to
comply with the three-year medical practice requirement. The
plain language of the statute and traditional tools of statutory
construction compel the conclusion that the Secretary’s regu-
lation is at odds with Congress’s clear intent in enacting the
Nursing Relief Act.

  The plain meaning of “was filed” is simply that application
was made.17 See Random House Unabridged Dictionary 531
(1973). It does not mean that an application was filed and
remains pending on a certain date.18 To require that the appli-
cation remained pending on a certain date imposes a new
requirement that is not contemplated by Congress.

   [6] As outlined above, Chevron’s first prong directs that, if
the statutory language is clear and unambiguous, the court
must give effect to the “plain meaning” of those words. See
Robinson v. Shell Oil Co., 519 U.S. 337, 340 (1997). Here,
Congress created a single condition precedent to the applica-
tion of the three-year, instead of the five-year, medical prac-
tice requirement: that an application for a national interest
waiver “was filed . . . prior to November 1, 1998.” See 8
U.S.C. § 1153(b)(2)(B)(ii)(IV). The Secretary’s regulation,
however, changes the statutory requirement with respect to an
entire class of doctors. It is plain to us that the Secretary’s
regulation conflicts with the Nursing Relief Act.
  17
      The instant case does not require us to conduct a detailed examination
of proper “filing,” such as timely delivery, fee submission, or date of
receipt. It is clear that the DHS received Dr. Tandar’s application and
accepted it for processing before November 1, 1998. Nothing in the record
suggests that the DHS denied Dr. Tandar’s application due to a defect
related to its filing; on the contrary, the record shows that DHS denied the
application on the merits.
   18
      As with “aggregate” in Part IV.A. above, the legal definition of “file”
is similar to the common definition. See Black’s Law Dictionary 642 (7th
ed. 1999) (“To deliver a legal document to the court clerk or record custo-
dian for placement into the official record.”).
6200                    SCHNEIDER v. CHERTOFF
   As a matter of common sense, the Secretary’s regulation
produces outcomes that contradict the plain language of the
statute. The statute clearly intends to subject to a three-year
medical practice requirement doctors whose employers
applied for a national interest waiver before November 1,
1998. The regulation, however, requires some of those doc-
tors instead to meet the five-year requirement.

   Dr. Tandar’s case is illustrative. An application for a
national interest waiver was filed on Dr. Tandar’s behalf
before November 1, 1998. Yet, the Secretary’s regulation
requires Dr. Tandar to complete the five-year medical practice
requirement. This anomalous outcome highlights the fact that
the regulation fails to give effect to the plain language of the
statute. The regulation plainly conflicts with Congress’s
clearly-expressed intent.

  [7] Because the Secretary’s implementing regulation is
contrary to Congress’s clear intent, we hold that this portion
of the regulation is in conflict with, and is ultra vires to, the
Nursing Relief Act.

  C.    The Four and Six-Year Limitations Periods on
        Completion of the Medical Practice Requirement

   [8] The Immigrant Doctors’ third claim challenges the reg-
ulation that limits the total time period in which an immigrant
doctor must complete the medical practice requirement. As
discussed above, the Nursing Relief Act provides that an
immigrant doctor who receives a national interest waiver can-
not obtain LPR status until the doctor has practiced in a short-
age area for an “aggregate” term of three or five years.19 See
8 U.S.C. § 1153(b)(2)(B)(ii)(II), (IV). The Secretary’s imple-
menting regulation conforms to the Act’s practice require-
ments, but imposes additional limitations periods.
Specifically, an immigrant doctor subject to the five-year
  19
    For the statutory text of the practice requirements, see note 13.
                      SCHNEIDER v. CHERTOFF                    6201
medical practice requirement must practice for an aggregate
of five years during the six-year period following the approval
of the immigrant visa petition and national interest waiver
request. See 8 C.F.R. § 204.12(b)(1) (“[T]he beneficiary phy-
sician must complete the aggregate 5 years of qualifying full-
time clinical practice during the 6-year period beginning on
the date of approval of the [immigrant visa petition].”). An
immigrant doctor subject to the three-year requirement must
practice for an aggregate of three years during the four-year
period following the approval of the immigrant visa petition
and national interest waiver request. See 8 C.F.R.
§ 204.12(d)(4) (“The physician must complete the aggregate
of 3 years of medical service within the 4-year period begin-
ning on the date of the approval of the [immigrant visa] peti-
tion . . . .”). That is, the Secretary’s regulation imposes a strict
time limit during which an immigrant doctor must complete
the medical practice requirement. The Immigrant Doctors
contend that the limitations period conflicts with the Nursing
Relief Act. We agree.

   Under the first prong of the Chevron approach, we again
ask whether Congress expressed a clear intent on the issue in
question. See Chevron, 467 U.S. at 842-43. If the statutory
language is clear and unambiguous, the court must give effect
to the “plain meaning” of those words. See Robinson, 519
U.S. at 340.

   Under the Nursing Relief Act, an immigrant doctor cannot
obtain LPR status “until such time as the alien has worked full
time as a physician for an aggregate of [3 or] 5 years.” 8
U.S.C. § 1153(b)(2)(B)(ii)(II), (IV). This requirement makes
sense. As discussed above, Congress passed the Nursing
Relief Act to “ensure[ ] that residents of areas with a shortage
of health care professionals will have access to quality health
care.” 145 Cong. Rec. H11,321, H11,322 (1999). To ensure
that an immigrant doctor contributes substantially to remedy-
ing the health professional shortage problem, Congress
refused to allow an immigrant doctor to obtain LPR status
6202                SCHNEIDER v. CHERTOFF
until the doctor had served a shortage community for three or
five years. Congress imposed no additional conditions prece-
dent to a grant of LPR status.

   [9] Congress clearly intended that no limitations period
should be imposed on the aggregate medical practice require-
ment. Congress did not impose a limitations period of its own
accord. Congress instead used particular language to the con-
trary — language that cannot be read to have left a “gap” for
the agency to fill. Congress restricted immigrant doctors from
obtaining LPR status “until such time” as the doctor had prac-
ticed in a shortage area for an “aggregate” of three or five
years. See 8 U.S.C. § 1153(b)(2)(B)(ii)(II), (IV) (emphasis
added). Congress did not place a limit on “such time,” demon-
strating that the time period would be open-ended. Further-
more, Congress used the word “aggregate” that invokes a
summation of fractured parts and contemplates interruptions.
See Part IV.A. Nothing in the Nursing Relief Act can be read
to authorize the Secretary to promulgate a regulation that
imposes a strict limitations period.

   We are confident that Congress would have imposed a lim-
itations period if it had so intended. Congress has crafted
detailed time limitations in other portions of the INA that
demonstrate its ability to make clear its intent with regard to
statutory time periods. See, e.g., 8 U.S.C. § 1229b(d)(2) (dis-
cussing breaks in ten years of continuous physical presence
for purposes of cancellation of removal, noting that any single
departure “in excess of 90 days or for any periods in the
aggregate exceeding 180 days” interrupt continuous pres-
ence); 8 U.S.C. § 1229c(b)(1), (2) (granting voluntary depar-
ture for a period not “exceeding 60 days” to certain aliens
who have been physically present in the United States for at
least one year before service of the charging document, and
who have had good moral character for at least five years); 8
U.S.C. § 1255a(a)(1) (granting adjustment of status to aliens
who entered the United States before January 1, 1982, pro-
vided that the alien apply for adjustment “during the 12-
                         SCHNEIDER v. CHERTOFF                         6203
month period beginning on a date (not later than 180 days
after November 6, 1986) designated by the Attorney Gener-
al,” but within thirty days of the service of any order to show
cause).

   The regulation, however, imposes a strict limitations period
not contemplated by the Nursing Relief Act. If an immigrant
doctor fails to comply with the regulatory limitations period,
the DHS will “deny the application for adjustment of status
and revoke approval of the [immigrant] visa petition and
national interest waiver.” 65 Fed. Reg. 53,889, 53,890 (Sept.
6, 2000). The regulation imposes on an immigrant doctor a
temporal obligation not required by statute.

  The Secretary argues that such a limitations period is nec-
essary. After all, absent a limitations period, the Nursing
Relief Act would potentially entitle an immigrant doctor to an
indefinite pending immigration status.20 The immigrant doctor
could lawfully engage in non-shortage medical practice with-
out penalty.

  The Secretary’s argument is well taken, but the fact
remains that Congress expressly permitted interruptions in
qualifying medical service by using the word “aggregate.”
Although the Secretary may believe that Congress made a
mistake by passing the law as it did, the Secretary cannot re-
  20
     This is so because, under the Nursing Relief Act, an immigrant doctor
can apply for adjustment of status based on the approved immigrant visa
petition and national interest waiver, but cannot obtain LPR status until the
doctor has met the medical practice requirement. If the immigrant doctor
were given an unlimited time to complete the medical practice require-
ment, the application for adjustment to LPR status would remain pending
until the immigrant doctor had completed the medical practice require-
ment. During the pendency of an application for adjustment to LPR status,
the immigrant doctor is authorized to remain in the United States and is
entitled to apply for employment authorization. See 8 C.F.R.
§ 274a.12(c)(9). Accordingly, the immigrant doctor could remain in the
United States working in any type of job until the doctor, at his leisure,
fulfills the medical practice requirement.
6204                 SCHNEIDER v. CHERTOFF
write the law. The Secretary is charged with “the administra-
tion and enforcement” of the INA and “shall establish such
regulations” as he deems necessary to enforce the INA. 8
U.S.C. § 1103(a)(1), (3). He may not, however, impose obli-
gations not required by law. The Secretary must defer to the
supremacy of Congress’s legislative enactments just as the
courts may not appropriate Congress’s legislative function.
“There is a basic difference between filling a gap left by Con-
gress’[s] silence and rewriting rules that Congress has affir-
matively and specifically enacted.” Lamie v. U.S. Trustee, 540
U.S. 526, 538 (2004) (internal quotation omitted); see also
United States v. Locke, 471 U.S. 84, 95 (1985) (“[D]eference
to the supremacy of the Legislature, as well as recognition
that Congressmen typically vote on the language of a bill,
generally requires us to assume that the legislative purpose is
expressed by the ordinary meaning of the words used.”)
(internal quotation omitted).

   [10] Accordingly, we hold that the Secretary’s regulation
that imposes a strict limitations period without statutory basis
conflicts with Congress’s clear intent, and is ultra vires to, the
Nursing Relief Act.

  D.   Specialist Doctors’ Eligibility for a National
       Interest Waiver

   In their fourth challenge to the regulations, the Immigrant
Doctors argue that certain specialist doctors are eligible for
national interest waivers under the Nursing Relief Act, but not
under the Secretary’s regulations. Congress instructed that a
national interest waiver “shall” issue to “any alien physician”
who agrees to work in a designated shortage area. See 8
U.S.C. § 1153(b)(2)(B)(ii)(I). The Secretary’s implementing
regulation states that only doctors who practice “in a medical
speciality that is within the scope of the Secretary’s designa-
tion for the geographical area or areas” will be eligible for a
national interest waiver. 8 C.F.R. § 204.12(a)(2)(i). The
                     SCHNEIDER v. CHERTOFF                    6205
Immigrant Doctors argue that the regulation conflicts with the
statute.

  We must first address the threshold question whether the
Immigrant Doctors have standing to bring this challenge. Dr.
Kasthuri, a radiologist, is the only specialist amongst the
Immigrant Doctors. Dr. Kasthuri has worked in a designated
shortage area since June 23, 2001. His employer filed an
immigrant visa petition and national interest waiver request
on his behalf on March 31, 2003. In his opening brief, Dr.
Kasthuri asserted that the applications were still pending.

   The Secretary argues that Dr. Kasthuri’s claim is moot,
because the DHS denied Dr. Kasthuri’s immigrant visa peti-
tion and national interest waiver request on August 9, 2004.
The Secretary avers that the DHS sent Dr. Kasthuri a request
for evidence to which Dr. Kasthuri did not respond, and that
the government accordingly denied his petition. In response to
an order of this court, the government produced the denial of
Dr. Kasthuri’s immigrant visa petition. The denial indicates
that Dr. Kasthuri did not respond to a request for additional
evidence, and that DHS therefore denied his application “due
to abandonment.”

   There is some dispute over whether Dr. Kasthuri ever
received the notice of denial of his immigrant visa petition.
But the question of notice is not before us. Our threshold
inquiry is whether Dr. Kasthuri, or any other Immigrant Doc-
tor, has standing to challenge this portion of the regulation.

   [11] “[F]ederal courts are without power to decide ques-
tions that cannot affect the rights of litigants in the case before
them.” North Carolina v. Rice, 404 U.S. 244, 246 (1971); see
also U.S. Const. art. III, § 2, cl. 1. The inability of the federal
judiciary “to review moot cases derives from the requirement
of Article III of the Constitution under which the exercise of
judicial power depends upon the existence of a case or contro-
versy.” Liner v. Jafco, Inc., 375 U.S. 301, 306 n.3 (1964). The
6206                 SCHNEIDER v. CHERTOFF
“irreducible constitutional minimum of standing contains
three elements”: (1) injury in fact; (2) causation; and (3) like-
lihood that a favorable decision will redress the injury. Lujan
v. Defenders of Wildlife, 504 U.S. 555, 560 (1992).

   [12] The DHS denied Dr. Kasthuri’s petition because of
abandonment, not because the DHS found Dr. Kasthuri ineli-
gible for a national interest waiver. Dr. Kasthuri’s argument
that the regulation caused his injury must fail. Similarly, a
favorable ruling on the regulatory challenge would not redress
Dr. Kasthuri’s injury; any decision rendered by this court on
the issue would be an advisory opinion that would have no
impact on the rights of the parties. “It has long been settled
that we have no authority to give opinions upon moot ques-
tions or abstract propositions, or to declare principles or rules
of law which cannot affect the matter in issue in the case
before us.” DHX, Inc. v. Allianz AGF MAT, Ltd., 425 F.3d
1169, 1174 (9th Cir. 2005) (internal quotation omitted). Dr.
Kasthuri’s claim became moot when his immigrant visa peti-
tion and national interest waiver request were denied because
of abandonment. We therefore dismiss this part of the appeal
and decline to reach the merits of the claim.

  E.   Reporting Compliance with the Nursing Relief Act

   Fifth, the Immigrant Doctors allege that the Secretary’s
implementing regulation imposes a double compliance system
that is contrary to the statute. Specifically, the Immigrant
Doctors argue that a doctor should not be required to apprise
the DHS twice about the progress that he or she has made
towards the medical practice requirement.

   [13] Under the Secretary’s regulation, immigrant doctors
subject to the five-year medical practice requirement must
submit certain evidence within 120 days of the completion of
the second year of the medical practice requirement, see 8
C.F.R. § 245.18(g)(1), and additional evidence within 120
days of completing the fifth year of the medical practice
                       SCHNEIDER v. CHERTOFF                       6207
requirement, 8 C.F.R. § 245.18(h). If an immigrant doctor
fails to comply, the DHS will first serve the doctor with a
notice of intent to deny the application for adjustment to LPR
status. See 8 C.F.R. § 245.18(i). Absent evidence demonstrat-
ing compliance, the DHS will then deny the application for
LPR status and revoke the approved immigrant visa petition
and national interest waiver. See id.

   [14] In contrast to Parts IV.A. - IV.C. above, the Nursing
Relief Act does not discuss compliance with the medical prac-
tice requirement that it prescribes. Congress was silent regard-
ing compliance. Yet, as with any other benefit conferred
under the INA, Congress surely contemplated that applicants
would be required to submit evidence of their entitlement to
the accelerated path to LPR status. The statute thus left a
“gap” within the meaning of Chevron’s first prong to be filled
by the agency. See Defenders of Wildlife v. Browner, 191 F.3d
1159, 1162 (9th Cir. 1999), amended by 197 F.3d 1035 (9th
Cir. 1999). Because the statute is silent with respect to the
issue at hand, we ask only whether the Secretary’s implement-
ing regulation is not an arbitrary or capricious but a permissi-
ble construction of the statute. See Wilderness Soc’y, 353 F.3d
at 1059 (citing Chevron, 467 U.S. at 843). We must defer to
the regulation unless the Secretary’s interpretation is contrary
to clear congressional intent or frustrates the policy Congress
sought to implement. See Biodiversity Legal Found. v. Badg-
ley, 309 F.3d 1166, 1172 (9th Cir. 2002).

   [15] The Secretary determined that immigrant doctors
would be required to submit evidence of their compliance
with the statutory scheme under the Nursing Relief Act. See
8 C.F.R. § 245.18(g), (h). It is eminently reasonable for the
Secretary to require multiple submissions of evidence from
the immigrant doctor if the immigrant doctor is subject to the
lengthy five year medical practice requirement.21 The five-
  21
    We observe that Immigrant Doctors subject to the three-year medical
practice requirement are only required to submit evidence once. See 8
C.F.R. § 245.18(g)(2). It thus appears that the Nursing Relief Act’s
lengthy five-year medical practice requirement animated the need for dou-
ble compliance.
6208                 SCHNEIDER v. CHERTOFF
year medical practice requirement is longer than many other
statutory periods in the INA, and its length justifies the multi-
ple compliance requests. See, e.g., 8 C.F.R. § 216.2(b)
(requiring conditional permanent residents to submit addi-
tional evidence “90 days before the second anniversary” of
the grant of conditional permanent resident status where statu-
tory period is only two years).

   [16] We cannot say that the compliance system devised by
the Secretary is in any way contrary to congressional intent or
that it frustrates a policy Congress sought to implement. The
Immigrant Doctors cited virtually no authority to the contrary.
Because the Secretary’s compliance period is a permissible
construction of the statute that is not arbitrary and capricious,
we hold that the compliance system is not in conflict with,
and is not ultra vires to, the Nursing Relief Act.

V.     CONCLUSION

   For the foregoing reasons we hold that the following por-
tions of the Secretary’s regulation are in conflict with, and are
ultra vires to, the Nursing Relief Act: (1) the exclusion from
the medical practice requirement of shortage-area medical
practice that occurs before approval of the immigrant visa
petition and national interest waiver; (2) the requirement that
doctors who applied for a national interest waiver before
November 1, 1998, but whose applications were denied
before November 12, 1999, comply with the five-year medi-
cal practice requirement; and (3) the requirement that immi-
grant doctors complete the medical practice requirement
within a four or six-year limitations period. We hold that the
Immigrant Doctors lack standing to challenge the portion of
the regulations that exclude specialist physicians like Dr.
Kasthuri from obtaining a national interest waiver. Finally, we
hold that the regulation that creates an alleged “double com-
pliance” system is a permissible construction of the Nursing
Relief Act that is not ultra vires to the Act.
                    SCHNEIDER v. CHERTOFF                 6209
  We AFFIRM in part and REVERSE in part, and
REMAND to the district court with instructions to enter
declaratory and injunctive relief for the plaintiffs consistent
with this opinion.
