a) According to the law of conservation of momentum, when two bodies act on one another, their total momentum remains constant until and unless there is no external force acting on them. Momentum can neither be created nor destroyed.
i) When a rocket is taking off from the ground, the chemicals in the fuel is burnt producing a very high velocity. This gas is released from the nozzle that is present in the tail pushing the rocket in the downward direction with a speed which pushes the rocket upwards by balancing the momentum of the gases.
ii) An equal and opposite momentum is experienced by the jet aeroplanes as the fuel used burns providing a high velocity to the jet.