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 air 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 warn air rises, cold air rushes in to take its place. 'Thus winds originate (起源). The 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 the causes of

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

Air moves under such conditions that

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

Which of the following pictures best tells the movements of warm air and cold air?
According to the passage, in same areas if farmers who grow grapes (葡萄) light fires in the early morning, that is because the farmers want to________________．

A．drive away warm air	B．prevent cold air from coming to harm their plants
C．cause more wind	D．stop warm air running away