Problem

There are four concentric shells *A, B, C* and *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

\left( k = \frac{1}{4\pi\epsilon_0} \right)

(a)

\frac{kq}{2a}

(b)

\frac{kq}{3a}

(c)

\frac{kq}{4a}

(d)

\frac{kq}{6a}