Find the Exact Value of the Expression Using the Provided $\cos ( s - t )$

Find the exact value of the expression using the provided information.
-Find $\cos ( s - t )$ given that $\cos s = \frac { \sqrt { 5 } } { 5 }$ , with $s$ in quadrant $I$ , and $\sin t = - \frac { \sqrt { 3 } } { 2 }$ , with $t$ in quadrant IV.

A) $- \frac { \sqrt { 5 } } { 10 }$

B) $\frac { \sqrt { 5 } + 2 \sqrt { 15 } } { 10 }$

C) $\frac { \sqrt { 5 } - \sqrt { 15 } } { 9 }$

D) $\frac { \sqrt { 5 } - 2 \sqrt { 15 } } { 10 }$