Two small and heavy spheres, each of mass M, are placed a distance r apart on a horizontal surface. The gravitational field intensity at the mid-point of the line joining the centres of the spheres is

Gravitational field intensity at a point is defined as the gravitational force experienced by a unit mass placed al that point. Since the spheres have the same mass, the gravitational forces exerted by each sphere on a unit mass placed at the mid-point will be equal and opposite. Hence the gravitational field intensity at the mid-point is zero. Thus the correct choice is (a).