A solid conducting sphere of radius a having a charge q is surrounded by a concentric conducting spherical shell of inner radius 2a and outer radius as shown in figure . Find the amount of heat produced when switch is closed

Question

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