The angle between the pair of straight lines formed by joining the points of intersection of $x^2+y^2=4$ and $y=3x+c$ to the origin is a right angle. Then $c^2$ is equal to

Question

$$\begin {array} {1 1} (a)\;20 & \quad (b)\;13 \\ (c)\;\frac{1}{5} & \quad (d)\;5 \end {array}$$