A solid sphere of uniform density and radius 4 units is located with its centre at origin O of coordinates. Two spheres of equal radii 1 unit, with their centres at A (-2, 0, 0) and B (2, 0, 0) respectively are taken out of the solid sphere leaving behind spherical cavities as shown in Fig. Choose the incorrect statement from the following. 

The distance of each cavity from the centre O is the same. Since the two cavities are symmetrical with respect to the centre O and the mass of the sphere can be regarded as being concentrated at the centre O , the gravitational force due to the sphere is zero at the centre. Hence choice (a) is correct. For the same reason, the gravitational potential is the same at all points of the circle y² + z² = 36 whose radius is 6 units and at all points of the circle y² + z² = 4 whose radius is 2 units. Hence choices ( c) and ( d) are also correct. But the gravitational force at point B cannot be zero.