Scallops use jet propulsion to move from one place to another. Their shells make them denser than water, so they normally rest on the ocean floor.
If a scallop wishes to remain stationary, hovering a fixed distance above the ocean floor, it must eject water so that the thrust force on the scallop is .

a) upward; upward
b) upward; downward
c) downward; upward
d) downward; downward

c

Explanation:

Since he shells of scallops make them denser than water, they normally rest on the ocean floor, which means there is a force needed to overcome the gravitational pull, which pullls them towards the ocean flootr. To hover in the water they must use their yet propulsion to push upwards ( otherwise they sink to the floor again).

To be able to create an upward force, the yet propulsion needs to eject water downwards. Every action has the same and opposite reaction (Third law of Newton).

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