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 gasoline and thus enabled scientists to differentiate (分區) 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 North-eastern 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.

　　62. 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

　　63. Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US ________.

　　A) was discouraged C) was prohibited by law

　　B) was enforced by law D) was introduced

　　64. 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.