The common tangent to the parabola $y^2=4ax$ and $x^2=4ay$ is

Question

$\begin{array}{1 1}(A)\;x+y+a=0 \\(B)\;x+y-a=0 \\(C)\;x-y+a=0 \\(D)\;x-y-a=0 \end{array}$