Find the Expression as the Sine or Cosine of an Angle.​

Multiple Choice

Find the expression as the sine or cosine of an angle.​ $$\cos 9 x \cos 5 y + \sin 9 x \sin 5 y$$ ​

A) $\sin ( 5 x - 9 y )$
B) $\sin ( 9 x - 5 y )$
C) $\cos ( 5 x - 9 y )$
D) $\cos ( 9 x - 5 y )$
E) $\cos ( 9 x + 5 y )$