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随时随地学习
随时随地学习
Air is around us. W can not s it, smll it, or tast it. But w can fl th wind blow. W can s th wind mov wavs on th watr, clouds in th sky, and tr branchs. Wind is moving air.
Without air w could not brath. Thr could b no living plants or animals. Bcaus sound travls through air, without air thr would b silnc. Th movmnt of air can support a larg, havy plan.
Air is a mixtur of gass and watr vapor. Th most important gass in th air ar nitrogn and oxygn. 78 prcnt of th air is mad up of nitrogn, and about 21 prcnt of oxygn. Almost all living things us th oxygn in air. Fir cannot burn without oxygn.
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