In an arrangement of two concentric conducting shells, with centre at origin and radii a and b(a < b), charges Q1 and Q2 are given to inner and outer shell, then the potential V at a distance r from the origin is (r < a) A) kQ1/A + kQ2/b B) KQ1/r + kQ2/a C) KQ1/a + KQ2/r D) k(Q1+Q2)/r