Two objects in space are exerting a certain amount of gravitational force on each other. Something causes the distance between the two objects to change, so now they are four times further apart. What is the resulting change in the force of gravity?
a) the force increases by a factor of 4
b) the force decreases by a factor of 4 (1/4 the force)
c) the force increases by a factor of 16
d) the force decreases by a factor of 16 (1/16 the force)