There are four concentric shells A , B , C ,D of radii a , 2a ,3a and 4a respectively . Shells B and D are given charges +q and -q respectively . Shell C is now earthed . The potential difference $\;V_{A}-V_{C}\;$ is

Question

$(a)\;\large\frac{k\;q}{2a}\qquad(b)\;\large\frac{k\;q}{3a}\qquad(c)\;\large\frac{k\;q}{4a}\qquad(d)\;\large\frac{k\;q}{6a}$