Find the Expression as the Sine or Cosine of an Angle

Multiple Choice

Find the expression as the sine or cosine of an angle. $$\cos 6 x \cos 3 y + \sin 6 x \sin 3 y$$

A) $\cos ( 6 x - 3 y )$
B) $\sin ( 6 x - 3 y )$
C) $\cos ( 6 x + 3 y )$
D) $\sin ( 3 x - 6 y )$
E) $\cos ( 3 x - 6 y )$