Find the Exact Value of the Expression Using the Provided $\cos ( \mathrm { s } + \mathrm { t } )$

Multiple Choice

Find the exact value of the expression using the provided information.
-Find $\cos ( \mathrm { s } + \mathrm { t } )$ given that $\sin \mathrm { s } = - \frac { 1 } { 2 }$ , with $\mathrm { s }$ in quadrant IV, and $\sin \mathrm { t } = \frac { 1 } { 4 }$ , with $\mathrm { t }$ in quadrant II.

A) $\frac { 1 - 3 \sqrt { 5 } } { 8 }$
B) $\frac { 4 \sqrt { 3 } - \sqrt { 15 } } { 11 }$
C) $- \frac { 4 \sqrt { 3 } + \sqrt { 15 } } { 11 }$
D) $\frac { 1 + 3 \sqrt { 5 } } { 8 }$