Passage Three 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 world-wide 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
gasolines and thus enabled 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 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
(生态系统 ) 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 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 Medical Center's study?
A) Controlling one's calorie intake is more important than doing
exercise.
B) Even occasional exercise can help reduce weight.
C) Weight reduction is impossible without exercise.
D) One could lose ten pounds in a year's time if there's no increase in
food intake.
30. What is the author's purpose in writing this article?
A) To justify the study of the Boston University Medical Center.
B) To stress the importance of maintaining proper weight.
精彩生活 
