Two stars of equal mass rotate about a common center of mass in a common circular orbit of radius. The total mass of the stars is the gravitational force exerted by the stars on each other is:
(a) Directly proportional to the square of the radius
(b) Inversely proportional to the square of the radius
(c) Directly proportional to the cube of the radius
(d) Inversely proportional to the cube of the radius
