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  3. For the first two-thirds of the 20th century, chemistry was seen by many as the science of the future. The potential of chemical

For the first two-thirds of the 20th century, chemistry was seen by many as the science of the future. The potential of chemical

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问题    For the first two-thirds of the 20th century, chemistry was seen by many as the science of the future. The potential of chemical products for enriching society appeared to be unlimited. Increasingly, however, and especially in the public mind, the negative aspects of chemistry have come to the fore, Disposal of chemical by- products at waste-disposal sites of limited capacity has resulted in environmental and health problems of enormous concern. The legitimate use of drugs for the medically supervised treatment of diseases has been tainted by the growing misuse of mood-altering drugs. The very word chemicals has come to be used all too frequently in a pejorative sense. There is, as a result, a danger that the pursuit and application of chemical knowledge may be seen as bearing risks that outweigh the benefits.
   It is easy to underestimate the central role of chemistry in modern society, but chemical products are essential if the world’s population is to be clothed, housed, and fed. The world’s reserves of fossil fuels (e. g. oil, natural gas, and coal) will eventually be exhausted, some as soon as the 21st century, and new chemical processes and materials will provide a crucial alternative energy source. The conversion of solar energy to more concentrated, useful forms, for example, will rely heavily on discoveries in chemistry. Long-term, environmentally acceptable solutions to pollution problems are not attainable without chemical knowledge. There is much truth in the aphorism that "chemical problems require chemical solutions". Chemical inquiry will lead to a better understanding of the behaviour of both natural and synthetic materials and to the discovery of new substances that will help future generations better supply their needs and deal with their problems.
    Progress in chemistry can no longer be measured only in terms of economics and utility. The discovery and manufacture of new chemical goods must continue to be economically feasible but must be environmentally acceptable as well. The impact or new substances on the environment can now be assessed before large-scale production begins, and environmental compatibility has become a valued property of new materials. For example, compounds consisting of carbon fully bonded to chlorine and fluorine, called chlorofluorocarbons (or Freons), were believed to be ideal for their intended use when they were first discovered. They are nontoxic, nonflammable gases and volatile liquids that are very stable. These properties led to their widespread use as solvents, refrigerants, and propellants in aerosol containers. Time has shown, however, that these compounds decompose in the upper regions of the atmosphere and that the decomposition products act to destroy stratospheric ozone. Limits have now been placed on the use of chlorofluorocarbons, but it is impossible to recover the amounts already dispersed into the atmosphere.
   The chlorofluorocarbon problem illustrates how difficult it is to anticipate the overall impact that new materials can have on the environment. Chemists are working to develop methods of assessment, and prevailing chemical theory provides the working tools. Once a substance has been identified as hazardous to the existing ecological balance, it is the responsibility of chemists to locate that substance and neutralize it, limiting the damage it can do or removing if from the environment entirely.  The last years of the 20th century will see many new, exciting discoveries in the processes and products of chemistry, Inevitably, the harmful effects of some substances will outweigh their benefits, and their use will have to be limited. Yet, the positive impact of chemistry on society as a whole seems beyond doubt.
The third paragraph is mainly about ______.
选项 A、the harmful effects of chlorofluorocarbons on the environment
B、the necessity to ensure the environmental compatibility of new chemical substances
C、the economic feasibility of new chemical substances
D、the objective assessment of the progress in chemistry
答案B
解析 该题问：第三段的主旨是什么?本题要求判断第三段的主旨。第三段前两句说：“不能仅从经济和实用性这两个角度来衡量化学所取得的进步。发明和生产新的化学产品在经济上必须一直有可行性，但是，还要使其与环境相适应。”接着，该段以氟利昂对环境产生的危害为例，说明了化学产品必须与环境相适。所以，选项B的the necessity to ensure the environmental compatibility of new chemical substances(确保化学物质和环境相适的必要性)正确，应为答案。其他三个选项A项意为“氯氟碳化合物对环境的有害影响”；C项意为“新化学产品经济角度的可行性”；D项意为“客观评估化学取得的进步”，所涉及的都只是文章的局部意思，不能概括全文大意，不是答案。
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