(a) No person shall be admitted to competitive examination unless such person is a citizen or national of the United States.

(b) No person shall be given any appointment in the competitive service unless such person is a citizen or national of the United States.

(c) OPM may, as an exception to this rule and to the extent permitted by law, authorize the appointment of aliens to positions in the competitive service when necessary to promote the efficiency of the service in specific cases or for temporary appointments.