What might have been discussed before this passage（）

A．The volume of the atmosphere is four times that of the world's.
B．The water in the oceans is the main source of rain and snow on land areas.
C．The atmosphere is mainly composed of the vapor carded over land by air currents.
D．The earth cannot support the water in the atmosphere if it falls down onto the earth suddenly.

A．There is little water contained in the atmosphere even in a heavy rainstorm.
B．In some places there falls an amount of over 400 inches of rain every year.
C．Most water vapor comes from the Oceans all over the world.
D．The process of water returning to the atmosphere is called evaporation.