The net external torque on a system of particles about an axis is zero. Which of the following are compatile with it ?
(a) The forces may be acting radially from a point on the axis
(b) The forces may be acting on the axis of rotation
(c )The force may be acting parallel to the axis of rotation
(d) The torque caused by some forces may be equal and opposite to that caused by other forces.

Options

A. a,b only
B. b,c only
C. a,d only
D. All

Answer

D