Find the Exact Value of the Function $\frac { 1 } { 2 }$

Multiple Choice

Find the exact value of the function. Rationalize the denominator if necessary.
-cos 240°

A) $\frac { 1 } { 2 }$
B) $- \frac { 1 } { 2 }$
C) $- \frac { \sqrt { 3 } } { 2 }$
D) $\frac { \sqrt { 2 } } { 2 }$