Correlation coeffiecient between $x$ and $y$ is $0.5 \: cov ( x,y )=8$ and $\sigma_y=4$ then $\sigma_x$ is

Question

$\begin {array} {1 1} (A)\;2 & \quad (B)\;6 \\ (C)\;4 & \quad (D)\;None\: of \: these \end {array}$