单选题

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.

单选题 ______ cannot live without oxygen.

【正确答案】 D

【答案解析】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。

单选题 Pure oxygen is provided for ______ .

【正确答案】 D

【答案解析】本题题干问“纯氧经常是提供给谁”。原文第1段第3句话指出医院里的病人和在山区工作的研究人员需要纯氧。因此选项D．both A and C是正确答案。

单选题 Breathing is difficult ______ .

A、 when you get to the top of a high mountain
B、 when you try to climb a high mountain
C、 when you fly in a plane
D、 when you get to the foot of a high mountain

【正确答案】 A

【答案解析】本题题干问“什么时候呼吸会变得困难”。原文第2段第2句话指出在山顶上呼吸会变得困难。因此选项A，when you get to the top of a high mountain是正确答案。

单选题 Which is more important for the burning of a fire?

【正确答案】 B

【答案解析】本题题干问“对于燃烧来说什么更加重要”。原文第3段指出空气中有足够的氧气可供燃烧，纯氧会让燃烧变快导致浪费。因此选项B．Air是正确答案。

单选题 Which of the following is NOT TRUE?

A、 The higher it is, the thinner the air would be.
B、 The higher it is, the less oxygen you will feel.
C、 The higher you arrive, the more plentiful oxygen you will breathe.
D、 The higher it is, the more difficult you will breathe.

【正确答案】 C

【答案解析】本题测试辨认正确信息的能力。原文第2段第1句话指出高处的空气变得稀薄。因此选项C．The higher you arrive, the more plentiful oxygen you will breathe，与原文意思相反，是正确答案。