The word "it" in the sentence "seawater directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared." (paragraph 4,line 4) refers to______.

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.

A．runoff
B．precipitation
C．evaporation
D．melting