Which of the following is/are correct?
A. If centre of mass of three particles is at rest and it is known that two of them are moving along different lines then the third particle must also be moving.
B. If centre of mass remains at rest then the net work done by the forces acting on the system must be zero.
C. If the centre of mass remains at rest then the net external force must be zero.
D. If the velocity of the centre of mass is changing then there must be some net work being done on the system
