C
Congress enacted a law requiring all civil service employees to retire at age 75 , except when such employees are employed by the armed services. Civil service employees of the armed services are required to retire at age 65 . Portman is an employee of the Department of the Army; he is 65 years old. He files suit in the federal district court seeking a declaratory judgment that would prevent the Army from requiring him to retire before age 75 . Portman's strongest argument in support of his contention that the statute's provisions regarding civil service employees of the armed services are invalid is that this provision:
A. Denies him the privileges and immunities of national citizenship. 
B. Denies him a property right without just compensation.
C. Is invidious discrimination on the basis of age in violation of the Fifth Amendment.
D. Is not within the enumerated powers of Congress under Article I, Section 8.