There are two concentric spherical shells of radii r and 2r . Initially a charge Q is given to the inner shell . Now , switch $\;S_{1}\;$ is closed and opened then $\;S_{2}\;$ is closed and opened and the process is repeated n times for both the keys alternatively . Find the final potential difference between the shells

Question

$(a)\;\large\frac{kQ}{2^{n} r}\qquad(b)\;\large\frac{kQ}{2^{n-1}r}\qquad(c)\;\large\frac{kQ}{r2^{n+1}}\qquad(d)\;\large\frac{kQ}{2r}$