Two identical small charged spheres are a certain distance apart, and each initially experiences an electrostatic force of magnitude f due to the other. with time, charge gradually diminishes on both spheres. when each of the spheres has lost half its initial charge, the magnitude of the electrostatic force will be

a) 1/16 f
b) 1/8 f
c) 1/2 f
d) 1/4 f