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  3. We know that oxygen is necessary for the breathing of animals and plants and for burning. Only one-fifth of air holds oxygen, bu

We know that oxygen is necessary for the breathing of animals and plants and for burning. Only one-fifth of air holds oxygen, bu

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问题    We know that oxygen is necessary for the breathing of animals and plants and for burning. Only one-fifth of air holds oxygen, but this is quite enough for all ordinary uses. Pure oxygen is used for special kinds of breathing and burning, for instance, patients in hospital and research workers working in high mountainous district.
   High up about the earth air is not so plentiful. On the top of a mountain our lungs would not take in as much oxygen at each breath as they would at the bottom, and on very high mountains, breathing is so difficult that men have little strength.
   Air has enough oxygen for the burning of a fire. Pure oxygen would cause the wood or coal to burn so fast that it would be wasted. When a very hot fire is needed for a special purpose, oxygen is supplied instead of air.
选项 A、Plants
B、Animals
C、Human beings
D、A, B and C
答案D
解析 此题题干问“没有了氧气什么不能存活”。在原文第1段第1句话可以得出动物和植物的呼吸都需要氧气，人也是高级动物，因此呼吸都需要氧气。因此选项D．A，B and C是正确答案。
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