Use an Identity to Write the Expression as a Single $\cos \left( \theta + \frac { \pi } { 2 } \right)$

Multiple Choice

Use an identity to write the expression as a single trigonometric function or as a single number.
- $\cos \left( \theta + \frac { \pi } { 2 } \right)$

A) $- \sin \theta$
B) $- \cos \theta$
C) $\sin \theta$
D) $\cos \theta$