Use Identities to Write Each Expression as a Function of $\cos ( s - t )$

Multiple Choice

Use identities to write each expression as a function of .
-Find $\cos ( s - t )$ given that $\cos s = \frac { \sqrt { 10 } } { 10 }$ , with $s$ in quadrant $I$ , and $\sin t = - \frac { \sqrt { 3 } } { 2 }$ , with $t$ in quadrant IV.

A) $\frac { \sqrt { 10 } - 3 \sqrt { 30 } } { 20 }$
B) $- \frac { \sqrt { 10 } } { 20 }$
C) $\frac { \sqrt { 10 } - \sqrt { 30 } } { 19 }$
D) $\frac { \sqrt { 10 } + 3 \sqrt { 30 } } { 20 }$