Simplify the Expression Algebraically $\cos ( 6 x + 9 y ) + \cos ( 6 x - 9 y )$

Multiple Choice

Simplify the expression algebraically.
$\cos ( 6 x + 9 y ) + \cos ( 6 x - 9 y )$

A) $\cos 6 x$
B) $\cos 6 x \cos 9 y$
C) $2 \cos 6 x \cos 9 y$
D) $2 \cos 6 x$
E) $2 \cos x \cos y$