The passage mainly deals with ______.

A．is more common today than in the past decades
B．was more common in the old generations
C．increased ten times in the days of our parents and grandparents
D．afflicts only young people

A．Various factors combine to cause variations in the salinity of water.
B．As a result of the formation of ocean ice water salinity decreases.
C．The salinity of the water near the bottom of the oceans is relatively high.
D．in coastal areas the ocean is diluted by rivers.