The angle between the tangents draw from the origin to the parabola $y^2=4a(x-a)$ is

Question

$\begin{array}{1 1}(A)\;90^{\circ} \\(B)\;30^{\circ} \\(C)\;\tan^{-1}(1/2) \\(D)\;45^{\circ} \end{array}$