Whether a fish absorbs or loses water is based on the fact that all fish must maintain a certain amount of salt in their bodies to stay healthy. (34) Fish that live in fresh water have a higher concentration of salt in their bodies than the surrounding water. Consequently, water continuously flows into the fish's body to attempt to dilute the amount of salt in the fish until it is equal to the amount of salt in the surrounding water.
Ocean fish have the opposite problem. Surrounded by salt water, their bodies contain a relatively lower concentration of salt than the ocean water. In this case osmosis causes the fish to constantly lose water in order to equalize salt concentration inside and outside the fish. To partially compensate for the water loss, ocean fish actually drink water through their mouths. And (35) to get rid of the excess salt they take in by drinking seawater, they excrete some salt through cells in their gills.
(30)

A. Water absorbing ability.
B. The cell's membrane.
C. Its surroundings.
D. The ability to maintain salt.