Show That the Equation Is Not an Identity by Finding $\cos ( x + \pi ) = \cos x$

Multiple Choice

Show that the equation is not an identity by finding a value of x for which both sides are defined but not equal.
- $\cos ( x + \pi ) = \cos x$

A) 0
B) $\frac { \pi } { 2 }$
C) $- \frac { \pi } { 2 }$
D) $\frac { 3 \pi } { 2 }$