If $y=mx+c$ is a tangent to the ellipse $x^2+2y^2=6$ then $c^2$ is equal to

Question

$\begin{array}{1 1}(A)\;36/m^2 \\(B)\;6m^2-3 \\(C)\;3m^2-3 \\(D)\;6m^2+3 \end{array}$