Use a Sum or Difference Identity to Find the Exact $\tan \theta$

Multiple Choice

Use a sum or difference identity to find the exact value.
-Find $\tan \theta$ if $\sec \theta = \frac { \sqrt { 5 } } { 2 }$ and $\sin \theta < 0$ .

A) $- \frac { 1 } { 2 }$
B) 2
C) $- 2$
D) $- \sqrt { 5 }$