The force between the spheres increases when the mass increases in one of the spheres.
<u>Explanation:</u>
Newton law of universal gravity extends gravity beyond the earth's surface. This gravity depends directly on the mass of both objects and is inversely proportional to square of distance between their centers.
Since gravity is directly proportional to “mass of both interacting objects”, stronger objects with greater gravitational force attract. If the mass of one object increases, gravity between them also increases. For example, if an object's mass of one double, force between them also doubles.