Use the Graph to Complete the Identity $\cos 3 x \cos \frac { 3 } { 2 } x + \sin 3 x \sin \frac { 3 } { 2 } x =$

Multiple Choice

Use the graph to complete the identity.
- $\cos 3 x \cos \frac { 3 } { 2 } x + \sin 3 x \sin \frac { 3 } { 2 } x =$ ?

A) $y = \cos \frac { 3 } { 2 } x$
B) $y = \sin y = \cos \frac { 3 } { 2 } x$
C) $y = \cos y = \cos \frac { 9 } { 2 } x$
D) $y = \sin y = \cos \frac { 9 } { 2 } x$