题目内容

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. 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

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