Find the Expression as the Cosine of an Angle $\cos \frac { \pi } { 5 } \cos \frac { \pi } { 3 } - \sin \frac { \pi } { 5 } \sin \frac { \pi } { 3 }$

Multiple Choice

Find the expression as the cosine of an angle.
$\cos \frac { \pi } { 5 } \cos \frac { \pi } { 3 } - \sin \frac { \pi } { 5 } \sin \frac { \pi } { 3 }$

A) $\sin \frac { 15 \pi } { 8 }$
B) $\cos \frac { 15 \pi } { 8 }$
C) $\cos \frac { 8 \pi } { 15 }$
D) $\cos \frac { \pi } { 15 }$
E) $\sin \frac { 8 \pi } { 15 }$