# A and B are mutually exclusive events with $P(A) = \frac{1}{2} P(B)$ and $A \cup B = S$ the sample space, then $P(A)$ is equal to :
( A ) $\frac{1}{3}$
( B ) $\frac{1}{4}$
( C ) $\frac{2}{3}$
( D ) $\frac{3}{4}$