The word "they" (Line 5, Para. 4) refers to______.

A．The ocean salinity is lower in tropical regions.
B．The ocean salinity is higher in coastal regions where rivers flow into the sea.
C．The ocean salinity is higher in dry regions.
D．The ocean salinity is lower in regions with little precipitation.

A．Subtract water from the ocean through evaporation.
B．Add water to the ocean through precipitation.
C．Dissolve materials in the ocean through heating.
D．Convert water vapor into liquid water.