Use the Formula for the Cosine of the Difference of Two

Multiple Choice

Use the Formula for the Cosine of the Difference of Two Angles
- $\cos \left( 255 ^ { \circ } - 15 ^ { \circ } \right)$

A) $- \frac { 1 } { 2 }$
B) $- \frac { \sqrt { 3 } } { 2 }$
C) $\frac { 17 } { 4 }$
D) $\frac { \sqrt { 3 } } { 2 }$