【真題再現】

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 (北極的) 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 (區分) 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 (生態系統) 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)

【精講】

這是一篇典型報刊寫法的文章，文章開頭首先點出全篇論述的主題，把核心思想和關鍵信息和盤托出。

第一段提供的信息主要有兩點，一是六七十年代的鉛沉積的主要源頭是美國(primarily the result of leaded gasoline emissions originating in the United States)，二是考試*大]無鉛汽油在美國推廣后，全球范圍內的鉛沉積減少幅度很大(the lead accumulation worldwide has decreased significantly)。以后段落的內容全部圍繞這兩個信息點展開說明。

　　第二段是對第一段所提供的兩點信息進行具體地解釋，其中包括信息來源(第一個信息源于《自然》雜志)，研究機構(National Center for Scientific Research in France)，研究者(Dr. Charles Boutron)，以及信息細節(研究對象是格陵蘭的雪);第二個信息源于Boutron于1991年發表的另一項研究成果。

　　第三段是對第一個信息的說明，指出之所以把格陵蘭的雪中沉積的鉛歸因于美國的含鉛汽油，是因為美國與其他地區所使用的汽油成分比率不同，這里的雪中鉛的比率大部分符合美國汽油特點。

　　接下來的三個段落是對第二個信息的說明。首先是信息來源(《Ambio》雜志)和信息內容(lead levels in soil in the Northeastern United States had decreased markedly since the introduction of unleaded gasoline)，其次是研究結果發表之前許多科學家們的設想(Many scientists had believed that……)，最后是研究主要過程(examined samples of the upper layers of soil taken from the same sites of 30 forest floors in New England, New York and Pennsylvania)。

　　最后一段總結了這兩項研究成果的現實意義，指出其不應成為對污染不加控制的理由(should not be used as a license to pollute)。