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

Multiple Choice

Find the expression as the sine or cosine of an angle.​ $$\cos 100 ^ { \circ } \cos 60 ^ { \circ } - \sin 100 ^ { \circ } \sin 60 ^ { \circ }$$ ​

A) $\cos 40 ^ { \circ }$
B) $\sin 40 ^ { \circ }$
C) $\sin 160 ^ { \circ }$
D) $\cos 160 ^ { \circ }$
E) $\cos 100 ^ { \circ }$