Correct Option: A

The rifle and bullet have zero momentum and zero kinetic energy to start with. When the trigger is pulled, the bullet gains some momentum in the forward direction, but this is canceled by the rifle’s backward momentum, so the total momentum is still zero. The kinetic energies of the gun and bullet are both positive scalars, however, and do not cancel. The total kinetic energy is allowed to increase, because kinetic energy is being traded for other forms of energy. Some of the initial chemical energy of the gun powder, for example, is transformed into thermal energy and sound energy. From the relationship between kinetic energy and momentum, we know that if a massive particle and a light particle have the same momentum, the light one will have a lot more kinetic energy. If a light particle and a heavy one have the same velocity, the heavy one has more kinetic energy. Since the momentum of the gun and the bullet is same, the bullet which has a lower mass has a higher kinetic energy.