Considered as a continuous body of fluid, the atmosphere is another kind of ocean. Yet, in

view of the total amount of rain and snow on land areas in the course of a year, one of the most amazing water facts is the very small amount of water in the atmosphere at any given time. The volume of the lower seven miles of the atmosphere--the realm of weather events- is roughly four times the volume of the world's oceans. But the atmosphere contains very little water. It is chiefly in the form. of invisible vapor, some of which is carried over land by air currents. If all vapor suddenly fell from the air onto the earth's surface, it would form. a layer only about one inch thick. A heavy rainstorm on a given area may use up only a small percentage of the water from the air mass that passes over. How, then, can some land areas receive more than 400 inches of rain per year？ How can several inches of rain fall during a single storm in a few minutes or hours？ The answer is that rain yielding air masses are in motion, and as the driving air mass moves on, new moist air takes its place.

The basic source of most water vapor is the ocean, evaporation, vapor transport, and precipitation (陈雨) make up the continuous movement of water from ocean to atmosphere to land and back to the sea. Rivers return water the sea. In an underground arc (弧) of the cycle, flowing bodies of water discharge some water directly into rivers and some directly to the sea.

What might have been discussed before this passage？

A．The ocean.

B．The earth.

C．The rainfall.

D．The atmosphere.