Express the Product as a Sum Containing Only Sines or Cosines

Multiple Choice

Express the product as a sum containing only sines or cosines.
- $\cos ( 2 \theta ) \cos ( 4 \theta )$

A) $\frac { 1 } { 2 } [ \cos ( 2 \theta ) + \cos ( 6 \theta ) ]$
B) $\cos ^ { 2 } \left( 6 \theta ^ { 2 } \right)$
C) $\frac { 1 } { 2 } [ \cos ( 6 \theta ) - \cos ( 2 \theta ) ]$
D) $\frac { 1 } { 2 } [ \cos ( 6 \theta ) - \sin ( 2 \theta ) ]$