Objective question in Electrostatics

A charge Q is distributed over two concentric hollow spheres of radii r and(R>r)such that the surface densities are equal.The potential at the common centre a) frac{Q({{R}^{2}}+{{r}^{2}})}{4pi {{varepsilon }_{0}}(R+r)} b) frac{Q}{R+r} c) Zero d) frac{Q(R+r)}{4pi {{varepsilon }_{0}}({{R}^{2}}+{{r}^{2}})}