Solve the Problem $x , 0 \leq x \leq 2 \pi$

Solve the problem.
-For what numbers $x , 0 \leq x \leq 2 \pi$ , does $\cos x = 1$ ?

A) $0,2 \pi$
B) $\frac { \pi } { 2 }$
C) $\frac { \pi } { 2 } , \frac { 3 \pi } { 2 }$
D) none