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
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 comparing the chemical compositions 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 combat 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 long-term maintenance of body weight.Unfortunately, that message has not gotten through to the average American, who would rather try switching to "light" beer and low-calorie bread than increase physical exertion. The Centers for Disease Control, for example, found that fewer than one-fourth of overweight adults who were trying to shed pounds said they were combining exercise with their diet.
In rejecting exercise, some people may be discouraged too much by caloric-expenditure charts;for example, one would have to briskly walk three miles just to work off the 275 calories in one delicious Danish pastry (小甜饼). 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 President's Council on Physical Fitness and Sports.
Still, exercise's supporting role in weight reduction is vital. A study at the Boston University Medical Center of overweight police officers and other public employees confirmed that those who dieted without exercise regained almost all their old weight, while those who worked exercise into their daily routine maintained their new weight.
If you have been sedentary (极少活动的) and decide to start walking one mile a day, the added exercise could burn an extra 100 calories daily. In a year's time, assuming no increase in food intake, you could lose ten pounds. By increasing the distance of your walks gradually and making other dietary adjustments, you may lose even more weight.
26. What is said about the average American in the passage?
A) They tend to exaggerate the healthful effect of "light" beer.
B) They usually ignore the effect of exercise on losing weight.
C) They prefer "light" beer and low-calorie bread to other drinks and food.
D) They know the factors that play a positive role in keeping down body weight.
27. Some people dislike exercise because
A) they think it is physically exhausting
B) they find it hard to exercise while on a diet
C) they don't think it possible to walk 3 miles every day
D) they find consulting caloric-expenditure charts troublesome
28. "Even exercise professionals concede half a point here" (Line 3, Para. 2) means "They
A) agree that the calories in a small piece of pastry can be difficult to work off by exercise
B) partially believe diet plays a supporting role in weight reduction
C) are not fully convinced that dieting can help maintain one's new weight
D) are not sufficiently informed of the positive role of exercise in losing weight
29. What was confirmed by the Boston University Medica
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