(b) v0 = 0, a0 ≠ 0
(d) v0 ≠ 0, a0 ≠ 0
If a non-zero external force acts on a system of particles, it causes the centre of mass of the system to accelerate with acceleration a0 at any instant t. In such a case, the velocity of centre of mass of the system of particles is either v0 or zero.