Suppose X and Y Are Random Variables Is a Joint Density Function

Multiple Choice

Suppose X and Y are random variables. Find k such that the function $f ( x , y ) = \left\{ \begin{array} { c l } 0.1 e ^ { - ( \mid x + 0.4 y ) } & \text { if } x \geq 0 , y \geq 0 \\0 & \text { otherwise }\end{array} \right.$ is a joint density function.

A) 0.2
B) 2
C) 0.25
D) 1
E) 2.5
F) 0.4
G) 0.1
H) 0