Which of following statements are correct ? ltbgt (a) Centre of mass of a body always coincides with the centre of gravity of the body
(b) Central of mass of a body is the point at which the total garvitational torque on the body is zero
(c ) Couple on a body produces both trasnlational and rotation motion in a body
(d) Mechinical advantage greater than one means that small efforts can be used to lift a large load