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

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

$r = \large\frac{cov (x,y)}{\sigma_x \sigma_y}$
$0.5 = \large\frac{\not{8}^2}{\sigma_x \times \not{4}}$
$\sigma_x=\large\frac{2}{0.5}$
$\sigma_x=4$
Ans : (C)