Two identical conducting spheres, each of radius 2.0cm, are touching and are initially neutral. A charged insulating rod is brought near one sphere, inducing a charge separation. While the rod remains, the spheres are separated and placed 8.0cm apart (center to center). Measurements show that one sphere carries +4.0μC of charge and the other carries −4.0μC. What is the magnitude of the electrostatic force between the spheres?
a) 0.22N
b) 22N
c) 2.0N
d) 10N