Two particles, each carrying a charge whose magnitude equals that of a proton, are separated by a distance d and attract each other with force F. Which single change listed below will produce the greatest INCREASE in the magnitude of the force?
a) Double the magnitude of both charges, keeping the distance d unchanged.
b) Reduce the separation distance to 2d, keeping the charges unchanged.
c) Reduce the separation distance to 3d, keeping the charges unchanged.
d) Double the magnitude of one of the charges, keeping the distance d unchanged.