A boy is running to the east and jumps into a stationary boat in a lake. Once the boy lands in the boat, the boat moves to the east.

Momentum is conserved when the boy lands in the boat, assuming external forces are negligible. Initially, the stationary boat has zero momentum, so the total momentum is the boy’s momentum. After landing, the boy and boat move together; their combined momentum equals that initial momentum, even though their shared velocity is lower than the boy’s original velocity. Therefore, the initial momentum of the boy is equal to the momentum of the boy and boat, as choice D states.

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