Passage Three
Questions 21 to 25 are based on the following passage.
Lead deposits, which accumulated in soil and snow during the 1960’s and 70’s, were primarily the result of leaded gasoline emissions originating in the United States. In the twenty years that the Clean Air Act has mandated unleaded gas use in the United States, the lead accumulation worldwide has decreased significantly.
A study published recently in the journal Nature shows that air-borne leaded gas emissions from the United States were the leading contributor to the high concentration of lead in the snow in Greenland. The new study is a result of the continued research led by Dr. Charles Boutron, an expert on the impact of heavy metals on the environment at the National Center for Scientific Research in France. A study by Dr. Boutron published in 1991 showed that lead levels in arctic (北极的) snow were declining.
In his new study, Dr. Boutron found the ratios of the different forms of lead in the leaded gasoline used in the United States were different from the ratios of European, Asian and Canadian gasolines and thus enabled scientists to differentiate (区分) the lead sources. The dominant lead ratio found in Greenland snow matched that found in gasoline from the United States.
In a study published in the journal Ambio, scientists found that lead levels in soil in the Northeastern United States had decreased markedly since the introduction of unleaded gasoline.
Many scientists had believed that the lead would stay in soil and snow for a longer period.
The authors of the Ambio study examined samples of the upper layers of soil taken from the same sites of 30 forest floors in New England, New York and Pennsylvania in 1980 and in 1990. The forest environment processed and redistributed the lead faster than the scientists had expected.
Scientists say both studies demonstrate that certain parts of the ecosystem (生态系统) respond rapidly to reductions in atmospheric pollution, but that these findings should not be used as a license to pollute.
21. The study published in the journal Nature indicates that ________.
A) the Clean Air Act has not produced the desired results
B) lead deposits in arctic snow are on the increase
C) lead will stay in soil and snow longer than expected
D) the US is the major source of lead pollution in arctic snow(D)
22. Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US ________.
A) was discouraged
B) was enforced by law
C) was prohibited by law
D) was introduced(B)
23. How did scientists discover the source of lead pollution in Greenland?
A) By analyzing the data published in journals like Nature and Ambio.
B) By observing the lead accumulations in different parts of the arctic area.
C) By studying the chemical elements of soil and snow in Northeastern America.
D) By comparing the chemical compositions of leaded gasoline used in various countries.(D)
24. The authors of the Ambio study have found that ________.
A) forests get rid of lead pollution faster than expected
B) lead accumulations in forests are more difficult to deal with
C) lead deposits are widely distributed in the forests of the US
D) the upper layers of soil in forests are easily polluted by lead emissions(A)
25. It can be inferred from the last paragraph that scientists ________.
A) are puzzled by the mystery of forest pollution
B) feel relieved by the use of unleaded gasoline
C) still consider lead pollution a problem
D) lack sufficient means to combat lead pollution(C)
这是一篇典型报刊写法的文章，文章开头首先点出全篇论述的主题，把核心思想和关键信息和盘托出。第一段提供的信息主要有两点，一是六七十年代的铅沉积的主要源头是美国(primarily the result of leaded gasoline emissions originating in the United States)，二是考试*大]无铅汽油在美国推广后，全球范围内的铅沉积减少幅度很大(the lead accumulation worldwide has decreased significantly)。以后段落的内容全部围绕这两个信息点展开说明。
第二段是对第一段所提供的两点信息进行具体地解释，其中包括信息来源(第一个信息源于《自然》杂志)，研究机构(National Center for Scientific Research in France)，研究者(Dr. Charles Boutron)，以及信息细节(研究对象是格陵兰的雪);第二个信息源于Boutron于1991年发表的另一项研究成果。
第三段是对第一个信息的说明，指出之所以把格陵兰的雪中沉积的铅归因于美国的含铅汽油，是因为美国与其他地区所使用的汽油成分比率不同，这里的雪中铅的比率大部分符合美国汽油特点。
接下来的三个段落是对第二个信息的说明。首先是信息来源(《Ambio》杂志)和信息内容(lead levels in soil in the Northeastern United States had decreased markedly since the introduction of unleaded gasoline)，其次是研究结果发表之前许多科学家们的设想(Many scientists had believed that……)，最后是研究主要过程(examined samples of the upper layers of soil taken from the same sites of 30 forest floors in New England, New York and Pennsylvania)。
最后一段总结了这两项研究成果的现实意义，指出其不应成为对污染不加控制的理由(should not be used as a license to pollute)。
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