A nonzero external force acts on a system of particles. The velocity and the acceleration of the centre of mass are found to be u0 and a0 at an instant t. It is possible that

As a non-zero external force is present, so the state of motion will shange, so acceleration will definitely change irrespective of what previous state has been, i.e. whether the body was in rest or motion. So, B and D are correct.

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