Scallops use jet propulsion to move from one place to another. Their shells make them denser than water, so they normally rest on 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
I do not understand why must the answer be c. Why can't the answer be b?