Three metal spheres are placed next to each other as shown in the picture. a has two times the mass of b. c has one-third the mass of a. the distance between a and c is half that between a and b. given these conditions, which statement is true? a) the gravitational force between a and b is one-third that between a and c. b) the gravitational force between a and b is two-thirds that between a and c. c) the gravitational force between a and b is three-eighths that between a and c. d) the gravitational force between a and b is four- sevenths that between a and c.