The gas we are most familiar with is air. Since gases expand to fill any container, you might wonder why our air does not escape into outer space. The answer is simply that gravity holds the atmosphere close to the earth. In fact, due to the gravitational pull, the gases of our atmosphere accumulate near the surface of the earth. Air gets progressively thinner the higher you go, until it gradually disappears into space. Because the air is so thin at very high place, such as the Himalayas Mountain, climbers take oxygen to help them breathe.

(96 words)

　　What is the passage mainly about?
　　 A) The gravity of the earth.　　　　
　　 B) The gas around the earth — air.
　　 C) The surface of the earth.　　　　
　　 D) The air on the high place.
　　　　

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