A Random Variable X Is Exponentially Distributed with a Mean $\operatorname { Pr } ( 1 \leq X \leq 3 )$

A random variable X is exponentially distributed with a mean of 2. Find $\operatorname { Pr } ( 1 \leq X \leq 3 )$

A) $\frac { 1 } { 2 }$ ( $\mathrm { e } ^ { - 1 }$ - $\mathrm { e } ^ { - 3 }$ )
B) $\frac { 1 } { 2 }$ $e ^ { - 1 / 2 }$
C) $\frac { 1 } { 2 }$ $e ^ { - 3 / 2 }$
D) $e ^ { - 1 / 2 }$ - $e ^ { - 3 / 2 }$
E) none of these