Use the Sum-To-Product Formulas to Select the Sum or Difference $\cos 3 \theta + \cos 6 \theta$

Multiple Choice

Use the sum-to-product formulas to select the sum or difference as a product.
$\cos 3 \theta + \cos 6 \theta$

A) $2 \cos \frac { 9 \theta } { 2 } \cos \left( - \frac { 3 \theta } { 2 } \right)$
B) $2 \cos \frac { 9 \theta } { 2 } \sin \left( - \frac { 3 \theta } { 2 } \right)$
C) $2 \cos \frac { 9 \theta } { 2 } \cos \left( \frac { 3 \theta } { 2 } \right)$
D) $2 \cos \left( - \frac { 9 \theta } { 2 } \right) \cos \frac { 3 \theta } { 2 }$
E) $2 \cos \frac { 9 \theta } { 2 } \sin \left( \frac { 3 \theta } { 2 } \right)$