Write the Product as a Sum $\cos 7 x \cos 3 x$

Multiple Choice

Write the product as a sum. $\cos 7 x \cos 3 x$

A) $\cos 7 x + \cos 3 x$
B) $[ \cos 7 x + \cos 3 x ] / 2$
C) $[ \cos 10 x + \cos 4 x ] / 2$
D) $- [ \cos 10 x + \cos 2 x ] / 2$
E) none of these