If two tangents are drawn from a point on the circle $x^2+y^2=36$ to the circle $x^2+y^2=18$ then find the angle between the tangents.

Question

$\begin{array}{1 1}(a)\;45^{\large\circ}&(b)\;90^{\large\circ}\\(c)\;35^{\large\circ}&(d)\;120^{\large\circ}\end{array}$