Without the atmosphere, there would be no weather, no wind, rain, snow, or clouds. Air is
Air is a gas that expands (膨胀) when heated, gets lighter, and moves upward. When air is
cooled, it gets heavier, sinks close to the earth's surface, and flows like water in a great river. As warm air rises, cold air rashes to take its place. Thus winds originate (起源). Tile winds that blow high above us are caused by the warmer air running away from colder air. The wind we feel near the earth's surface is the heavy colder air trying hard to catch the warmer air. Changes in temperature cause the air to move. And of course there are many changes, so air movements are taking place practically all the time.
The first paragraph mainly tells us______.
A.why we don't feel air
B.why atmosphere is important to us
C.why there is air surrounding us
D.why there is air pressure