Line $y=mx+c$ will touch the parabola <Br> $\qquad \qquad \qquad x^2=4ay$ if,

Question

$\begin{array}{1 1}(A)\;c=am^2 \\(B)\;c=-am^2 \\(C)\;c=2am \\(D)\;none\;of\;these \end{array}$