Which of the following statements are correct?$\newline$(a) Centre of mass of a body always coincides with the centre of gravity of the body.$\newline$(b) Centre of mass of a body is the point at which the total gravitational torque on the body is zero.$\newline$(c) A couple on a body produces both translational and rotational motion in a body.$\newline$(d) Mechanical advantage greater than one means that small effort can be used to lift a large load.