阅读理解：
At the Kyoto conference on global warming in December 1997，it became abundantly clear how complex it has become to work out international agreements relating to the environment because of economic concerns unique to each country. It is no longer enough to try to forbid certain activities or to reduce emissions of certain substances. The global challenges of the interlink between the environment and development increasing bring us to the core of the economic life of states. During the late 1980s we were able，through international agreements，to make deep cuts in emissions harmful to the ozone layer. These reductions were made possible because substitutions had been found for many of the harmful chemicals and，more important，because the harmful substances could be replaced without negative effects on employment and the economies of states.
Although the threat of global warming has been known to the world for decades and all countries and leaders agree that we need to deal with the problem，we also know that the effects of measures，especially harsh measures taken in some countries，would be nullified（抵消）if other countries do not control their emissions. Whereas the UN team on climate change has found that the emissions of carbon dioxide would have to be cut globally by 60% to stabilize the content of CO，in the atmosphere，this path is not feasible for several reasons. Such deep cuts would cause a breakdown of the world economy. Important and populous（人口众多的）low-or medium-income countries are not yet willing to undertake legal commitments about their energy uses. In addition，the state of world technology would not yet permit us to make such a big leap.
We must，however，find a solution to the threat of global warming early in the 21st century. Such a commitment would require a degree of shared vision and common responsibilities new to humanity. Success lies in the force of imaginations，in imagining what would happen if we fail to act. Although many living in cold regions would welcome the global-warming effect of a warmer summer，few would cheer the arrival of the subsequent tropical diseases，especially where there had been none.
51. The main purpose of this passage is to .
A. analyze the problem of global warming
B. argue against making deep cuts in emissions
C. convince people that global warming is a real threat
D. criticize some countries for refusing to cut down emissions harmful to the ozone layer
52. The reason why it is difficult to get rid of the threat of global warming is that .
A. not all the countries are willing to make deep cuts in emissions
B. many people welcome the global-warming effect of a warmer summer
C. the leaders of many countries are not fully aware of the gravity of the problem
D. world technology is not able to solve the problem
53. In the passage the author implies that .
A. the world has recently become aware of the threat of global warming
B. the problem of global warming has largely been solved
C. it is always difficult to work out international agreements to cut down emission harmful to the ozone layer
D. it is no longer easy to reach international agreements relating to the environment
54. According to the author，it is impossible at present to cut 60% of carbon dioxide emissions globally because .
A. some people are irresponsible
B. it would cause a collapse of the world economy
C. it is only a goal to be reached in the future
D. some people are lacking in imagination
55. What should all countries do to help solve the problem of global warming？
A. They should hold another world conference on climate change.
B. They should provide advanced technology.
C. They should replace all the harmful substances.
D. They should willingly undertake legal commitments about their energy uses.
