[单项选择]

Without the atmosphere, there would be no weather, no wind, rain, snow, or clouds. Air is all around us; we live at the bottom of a great ocean of air. It is invisible, but we can feel it when it moves. There are miles of all above us, pressing down with great weight on the earth and everything on it. Because air presses down on us from all directions at the same time, and because we are able to bear (忍受) this weight, we do not seem to feel it. But scientists have proved that air has this weight, and that anything that has weight creates (产生) a force called pressure. As changes in air pressure take place, they make air move.
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). The winds that blow high above us are caused by the warmer air running away fr
A. there is air pressure
B. the air is heavy
C. air pressure doesn’t remain the same all the time
D. air comes up and down