[单项选择]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.
______ cannot live without oxygen.
A. Plants
B. Animals
C. Human beings
D. A, B and C

[单项选择]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.
Pure oxygen is provided for ______ .
A. research workers in high mountains
B. children
C. patients
D. both A and C

[单项选择]We know we have to read "between the lines" to get the most out of anything. Marking up is also a useful practice, (76) you shouldn’t mark up a book which isn’t yours. Librarians (77) lend you books expect you to keep them clean, and you should. If you decide that I am right about the usefulness of marking books, you will have to (78) them.
There are three kinds of book owners. The first has all the standard sets and best sellers—unread, (79) . The second has a (80) many books—few of them read (81) , most of them dipped into, but all of them as clean and shiny as the day they were bought. The third has a few books or many every one of them (82) and dilapidated.
Why is (83) a book indispensable to reading First, it keeps you awake. I mean wide (84) . In the second place, reading if it is active, is thinking, and thinking (85) express itself in words. Finally, wr
A. but
B. although
C. and
D. so

[填空题]It is necessary for us to know healthy weight is not ______.

[单项选择]From the conversation we know if we buy more products from the Universal Computers Ltd., ______.
A. we can earn more mone
B. we can benefit more
C. they will give us more service
D. they will move their main building to China