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

Multiple Choice

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

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