Two metal spheres of radius 5.0 cm carry net charges of + 1.0 C and + 0.2

Question:

Two metal spheres of radius 5.0 cm carry net charges of + 1.0 μC and + 0.2 μC.
(a) What (approximately) is the magnitude of the electrical repulsion on either sphere when their centers are 1.00 m apart?
(b) Why cannot Coulomb's law be used to find the force of repulsion when their centers are 12 cm apart?
(c) Would the actual force be larger or smaller than the result of using Coulomb's law with r = 12 cm? Explain.