Which of the following statements are correct?     [NEET 2017]
I. Centre of mass of a body usually coincides with the centre of gravity of the body.
II. Centre of mass of a body is the point at which the total gravitational torque on the body is zero.
III. A couple on a body produces both translational and rotational motion in a body.
IV. Mechanical advantage greater than one means that small effort can be used to lift a large load. 

Usually, the centre of gravity coincides with the centre of mass for bodies, when the gravitational field is uniform for bodies of small height. 
Since, the gravitational force can be taken to be centred or confined at the centre of gravity of body, so the torque of gravitational force about the centre of gravity must be zero. 
A couple always produces the rotation without translation. 

Mechanical advantage is the ratio of the force produced by a machine to the force applied on it. It is used in assessing the performance of a machine. So, option (2) is correct.