Which one of the following statements correctly compares the momentum vector diagrams of a two-body elastic collision when viewed (i) in the centre-of-mass frame and (ii) in the laboratory frame?
a) In the laboratory frame the momentum polygon closes, whereas in the centre-of-mass frame it remains open.
b) The sum of the individual momentum vectors is equal and opposite in the centre-of-mass frame at every instant.
c) The momentum vectors of the two bodies change magnitude but not direction in the centre-of-mass frame.
d) The total momentum is zero only after the collision in the centre-of-mass frame, but it is conserved at every instant in the laboratory frame.