Q. Two conducting spheres of radii 3 cm and 1 cm are separated by a distance of 10 cm in free space. If the spheres are charged to same potential of 10 V each, the force of repulsion between them is
(a) $\left(\frac{1}{3}\right)x {10}^{-9}N$                          (b) $\left(\frac{2}{9}\right)x {10}^{-9}N$
(c) $\left(\frac{1}{9}\right)x {10}^{-9}N$                         (d) $\left(\frac{2}{3}\right)x {10}^{-9}N$