A charge Q is distributed over two concentric conducting thin spherical shells radii r and R (R > r). If the surface charge densities on the two shells are equal, the electric potential at the common centre is :

(1) ${1\over4\pi\epsilon_0}{(2R+r)\over(R^2+r^2)}Q$

(2) ${1\over4\pi\epsilon_0}{(R+r)\over2(R^2+r^2)}Q$

(3) ${1\over4\pi\epsilon_0}{(R+2r)\over2(R^2+r^2)}Q$

(4) ${1\over4\pi\epsilon_0}{(R+r)\over(R^2+r^2)}Q$