If the slopes of the line of regression of $y$ on $x$ and of $x$ on $y$ are $30^{\circ}$ and $60^{\circ}$ respectively. Then $ r(x,y)$ is

Question

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