【正文】 e 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 ( 分區(qū) ) 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(tài)系統(tǒng) ) re spond 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 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 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 paring the chemical positions of leaded gasoline used in various countries. 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 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 bat lead pollution Passage Four Questions 26 to 30 are based on the following passage. Exercise is one of the few factors with a positive role in longterm maintenance of body , that message has not gotten through to the average American, who would rather try switching to light beer and lowcalorie bread than increase physical exertion. The Centers for Disease Control, for example, found that fewer than onefourth of overweight adults who were trying to shed pounds said they were bining exercise with their diet. In rejecting exercise, some people may be discouraged too much by caloricexpenditure charts。s study? A) Controlling one39。m enough to know it is going to be a very difficult situation to pete against three strong teams. A) realistic B) conscious C) register D) resolve 40. Can you give me even the clue as to where her son might be? A) simplest B) slightest C) least D) utmost 41. Norman Davis will be remembered by many with not only as a great scholar but also as a most delightful and faithful friend. A) kindness B) friendliness C) warmth D) affection 42. Salaries for positions seem to be higher than for permanent ones. A) legal B) optional C) voluntary D) temporary 43. Most people agree that the present role of women has already affected . society. it has affected the traditional role of men. A) Above all B) In all C) At most D) At last 44. Science and technology have in important ways to the improvement of agricultural production. A) attached B) assisted C)contributed D)witnessed 45. As an actor he could municate a whole of emotions. A) frame B) range C) number D) scale 46. This is what you should bear in mind: Don39。s generator broke down, and the pumps had to be operated instead of mechanically. A) artificially B) automatically C) manually D) synthetically 48. The little girl was so frightened that she just wouldn39。s time if there39。 Even exercise professionals concede half a point here. Exercise by itself is a very tough way to lose weight, says York Onnen, program director of the President39。s character 19. Free play has been introduced in some Japanese kindergartens in order to A) broaden children39。most Japanese preschools, surprisingly little emphasis is put on academic instruction. In one investigation, 300 Japanese and 210 American preschool teachers, child development specialists, and parents were asked about various aspects of early c