When the compressed spring is released, Blocks A and B will slide apart. There are three systems to consider here, indicated by the closed dashed lines below---System A, System B, and System A+B. Ignore the vertical forces of gravity and and the support force of the table. Does an external force act on System A? (yes) (no)
Will the momentum of System A change? (yes) (no)
Does an external force act on system B? (yes) (no)
Although the momentum of the bus is very large compared with the momentum of the bug, the change in momentum of the bus compared with the change of momentum the bug is (more) (the same) (less) .
Which undergoes the greater acceleration?
(bus) (both the same) (bug)
Which, therefore, suffers the greater damage?
(bus) (both the same) (the bug of course!)