题目内容
A．The natural colors of astronomical objects can be captured.B．The cost of equipment n
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Section B
Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
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 airborne 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 gasoline and thus enable scientists to differentiate (分区) 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 up per 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.
The study published in the journal Nature indicates that ______.
The AIDS epidemic is worse in American cities than in African cities.
Some experts believe that, if not for the cultural changes that Africa has experienced, the AIDS virus would not have spread to its present extent.
The penny press, which emerged in the United States during the 1830's, was a powerful agent of mass communication. These newspapers were little dailies, generally four pages in length, writ ten for the mass taste. They differed from the formal presentation of the conservative press, with its emphasis on political and literary topics. The new papers were brief and cheap, emphasizing sensational reports of police courts and juicy scandals as well as human interest stories. Twentiethcentury journalism was already foreshadowed in the penny press of the 1830's.
The New York Sun, founded in 1833, was the first successful penny paper, and it was followed two years later by the New York Herald, published by James Gordon Bennett. Not long after, Ho race Greeley issued the New York Tribune, which was destined to become the most influential paper in America. Greeley gave space to the issues that deeply touched the American people before tile Civil Warabolitionism, temperance, free homesteads, Utopian cooperate settlements, and the problems of labor. The weekly edition of the Tribune, with I00,000 subscribers, had a remarkable influence in rural areas, especially in Western communities.
Americans were reputed to be the most avid (热心的) readers of periodicals in the world. An English observer enviously calculated that, in 1829, the number of newspapers circulated in Great Britain was enough to reach only one out of every thirtysix inhabitants weekly; Pennsylvania in that same year had a newspaper circulation which reached one out of every four inhabitants weekly. Statistics seemed to justify the common belief that Americans were devoted to periodicals. Newspapers in the United States increased from 1,200 in 1833 to 3,000 by the early 1860's, on the eve of the Civil War. This far exceeded the number and circulation of newspapers in Eng land and France.
What is the author's main point in the first paragraph？
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