In the parabola $y^2=4ax$ , the length of the chord passing through the vertex and inclined to the x-axis at an angle $\theta$ is
$\begin{array}{1 1}(A)\;4a \cos \theta /\sin ^2 \theta \\(B)\;4a \sin \theta / \cos ^2 \theta \\(C)\;a \sec^2 \theta\\(D)\;a cosec ^2 \theta \end{array}$