Write the Product as a Sum or Difference of Trigonometric $2 \sin 3 x \sin 18 x$

Write the product as a sum or difference of trigonometric functions.
- $2 \sin 3 x \sin 18 x$

A) $\cos 21 x + \cos 15 x$
B) $\sin 21 x + \sin 15 x$
C) $\cos 15 x - \cos 21 x$
D) $\frac { 1 } { 2 } ( \cos 21 x + \cos 15 x )$