Express the Product as a Sum or Difference $\cos \frac { 7 x } { 2 } \cos \frac { x } { 2 }$

Multiple Choice

Express the product as a sum or difference.
- $\cos \frac { 7 x } { 2 } \cos \frac { x } { 2 }$

A) $\frac { 1 } { 2 } ( \cos 3 x + \cos 4 x )$
B) $\frac { 1 } { 4 } ( \cos 8 x - \sin 6 x )$
C) $\frac { 1 } { 4 } \cos ^ { 2 } 7 x$
D) $\frac { 1 } { 2 } ( \cos 4 x - \sin 3 x )$