$f ( x ) = \left\{ \begin{array} { l l } \frac { 1 } { 2 } e ^ { - x / 2 } & \text { if } x \geq 0 \\0 & \text { if } x < 0\end{array} \right.$

Multiple Choice

$f ( x ) = \left\{ \begin{array} { l l } \frac { 1 } { 2 } e ^ { - x / 2 } & \text { if } x \geq 0 \\0 & \text { if } x < 0\end{array} \right.$ is a probability density function for a particular random variable X. Use integration to find $P ( X \geq 4 )$

A) $e ^ { 2 }$
B) $1 + e ^ { 2 }$
C) $e ^ { - 2 } - 1$
D) $e ^ { - 2 }$