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Which of the following can be classified as a mechanical valve？
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According to the passage, with the increasing rate of Aids, ______.
人都说西湖的神韵在于______：三面春山如睡，中间盈着一汪湛湛碧水，真有“流出桃花波太软”的娴静；人又说西湖的神韵在于______，依依可人的苏堤白堤上，烟柳画桥，风帘翠幕，这份景致只有天宫仙境可比；人又说西湖的神韵在于______，她云山逶迤，亭台隐现，真有泼墨山水欲露还藏的意趣；人又说西湖的神韵在于她的_______，她那四时八节之景，一山一水一草一木，俱有灵秀之气，冬赏腊梅春折柳，而三秋桂子、十里荷花的美景，更让人流连沉醉。填人横线处最恰当的一项是（）。
That's not good enough, however, the same stability that makes CFC so safe in industrial use makes them extremely longlives, some of the CFCs released today will still be in the atmosphere a century from now. Moreover, each atom of chlorine liberated form. a CFC can break up as many as 100,000 molecules of ozone.
For this reason, governments should ensure the careful handling and recycling of the CFC now in use. When plastic -foam burger holders are broken, the CFCs trapped inside escape. Discarded refrigerators re lease CFCs as well, and, a significant part of the U.S. contribution to CFC emissions comes from draining automobile air conditioners. Such release of CFCs could be prevented if consumers and businesses were offered cash incentives to return broken down air conditioners and refrigerators to auto and appliance dealers. Then the units could be sent back to the manufacturers so that the CFCs could be reused.
While recycling will help, the only sure way to save the ozone is a complete ban on CFC manufacture, which should be phased out over the next five years. Fortunately, as the Montreal Protocal demonstrates, banning CFCs will be far simpler than reducing other dangerous gases. But a ban could admittedly be economically disruptive to the entire world: the annual market for CFCs is some $ 2.2 billion. The Soviet Union, which is a heavy user of CFCs, will have a particularly tough time phasing out the chemicals. "I agree with the ban in principle," said Vladimir Sakharov, a member of the Soviet State Committee for Enviromental Protection,'' but in practice it will be extremely difficult. Our economy is not flexible as others."
To make the transition easier, chemical companies are working hard to find practical substitutes for CF Cs. The most promising approach so far is to use CFC family members that are chemically altered to make them less dangerous to the environment. The chlorine - free substitutes is the 'high cost of making them. It may be that until better manufacturing techniques are developed, consumers will have to pay more for affect ed products. The prospect is not a pleasant one, it is a small price to pay for curbing the green house effect and saving the life -preserving ozone layer.
Why should governments ensure the careful handling and recycling of the CFCs now in use？
Criticism of research lays a significant foundation for future investigative work, but when students begin their own projects, they are likely to find that the standards of validity in field work considerably more rigorous than the standards for most library research. When students are faced with the concrete problem of proof by field demonstration, they usually discover that many of the "important relationships" they may have criticized other researchers for failing to demonstrate are very elusive indeed. They will find, if they submit an outline or questionnaire to their classmates for criticism, that other students make comments similar to some they themselves may have made in discussing previously published research. For example, student researchers are likely to begin with a general question but find themselves forced to narrow its focus. They may learn that questions whose meanings seem perfectly obvious to them are not clearly understood by others, or that questions which seemed entirely objective to them appear to be highly biased to someone else. They usually find that those who have not actually attempted it generally believe the formulation of good research questions is a much more subtle and frustrating task.
What does the author think about trying to find weaknesses in other people's research？
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