Express the Sum or Difference as a Product $\cos 4 x + \cos 2 x$

Multiple Choice

Express the sum or difference as a product.
- $\cos 4 x + \cos 2 x$

A) $2 \cos 3 x \cos x$
B) $2 \sin 3 x \sin x$
C) $2 \cos 3 x \sin x$
D) $2 \cos 3 x$