Rotational Motion (Moment of Inertia)

A solid sphere A of radius R and mass M is attached at a point to a smaller solid sphere B of radius r < R and mass m < M. The line joining their centres lies along the horizontal. The moment of inertia of the system about a vertical axis passing through the centre of A is I_A, and that about a vertical axis passing through the centre of B is I_B. The difference I_A - I_B is: