The value of K for which function $f(x) =\left\{ \begin{array}{l l} k\: e^{-3x} & \quad \text{ $x$ >0}\\ 0 & \quad \text{ elsewhere} \end{array} \right.$ is a Probability Distribution Function.

Question

$\begin {array} {1 1} (A)\;3 & \quad (B)\;\large\frac{1}{3} \\ (C)\;2 & \quad (D)\;-3 \end {array}$