Answer
Choice B.
Work Step by Step
Though the sand is dropped onto the boat vertically, it does affect the boat’s horizontal motion. Initially the sand’s horizontal velocity is zero. It must be accelerated from rest to the final horizontal velocity of the boat. There is a force between the boat deck and the sandbag, pushing the bag forward.
By Newton’s third law, the boat feels an equal but opposite force from the sand, directed backward, and this causes the boat speed to decrease.
