Identical conducting spheres carrying different charges attract each other with a force F when placed in a medium at a distance d apart. What is the nature of the relationship between F, d, and the charges?

a) F is directly proportional to the charges and inversely proportional to d.
b) F is inversely proportional to the charges and directly proportional to d.
c) F is directly proportional to both charges and d.
d) F is inversely proportional to both charges and d.