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 worldwide 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 (区分) 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 （）

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

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 worldwide 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 (区分) 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 (生态系统) respond rapidly to reductions in atmospheric pollution, but that these findings should not be used as a license to pollute.

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

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 worldwide 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 (区分) 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 (生态系统) respond rapidly to reductions in atmospheric pollution, but that these findings should not be used as a license to pollute.

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

Why is lead poisoning a new problem for condors

A:Hunters shoot condors. B:Puppet rearing practices have caused it. C:Hunter's bullets contain lead which poisons the meat condors eat. D:They never used to like lead.

How much lead is harmful Until the 1970s, exposure to lead was considered dangerous only at blood levels associated with extreme symptoms of poisoning: convulsions (惊厥), brains swelling, acute kidney disease, stomach pains and hallucinations (幻觉). But low -level lead poisoning can cause symptoms that, while subtle, are no less serious. As a result, the CDC has continually lowered the point at which patients should be treated for lead poisoning.
One of the most influential study of lead’ s low - level effects was published by Herbert Needleman, professor of psychiatry at the University of Pittsburgh Medical School. Testing first and second grade students in the Boston area, he found the lowest I. Q. , academic achievement, language skills and attention span among children with the highest tooth levels (起损害作用的量) of lead. "None of their lead levels exceed the CDC’s 1979 toxicity threshold. "Says Paul Mushak, an expert on toxic substances, "Our children and people in the risk groups shouldn’t be held hostage (成为人质) to a preventable disease. "
What can be done In some cases the treatment for low - level lead poisoning can be as simple as removing the source of pollution, for example, to stop drinking tap water in some cases. But no one knows the long - term effects of lead poisoning may be. The small amount of research done so far suggest that intellectual impairment may be irreversible (不可逆的).
Nor is it always easy to know what water is safe. Jeanne Briskin, a policy analyst with the EPA, reports that in uncounted schools across the country water in drinking fountains may be lead - contaminated. Even the child whose parents have removed the lead threat at home may still be gulping (吞咽)the metal every school day.
The American Academy of Pediatrics recommends that children be screened for lead poisoning at 12 months of age, with their doctor to determine follow - up examinations; men at risk for hypertension (高血压) should also have their blood lead levels tested.
Prevention is even more important. Public -health specialists have intensified efforts to control the pollutant. Meanwhile, for the individual, that means testing potential household sources of contamination.
One way to treat low -level lead poisoning is to

A:remove the source of pollution. B:to get rid of the lead in the body with medicine. C:stop drinking bottled water. D:stop eating foods that contain lea

Many theories concerning the causes of juvenile delinquency crimes committed by young people focus either on the individual or on society as the major contributing influence. Theories 16 on the individual suggest that children engage in criminal behavior 17 they were not sufficiently penalized for previous misdeeds or that they have learned criminal behavior through 18 with others. Theories focusing on the role of society suggest that children commit crimes in 19 to their failure to rise above their socioeconomic status 20 as a rejection of middleclass values. Most theories of juvenile delinquency have focused on children from disadvantaged families, 21 the fact that children from wealthy homes also commit crimes. The latter may commit crimes 22 lack of adequate parental control. All theories, however, are tentative and are 23 to criticism. Changes in the social structure may indirectly 24 juvenile crime rates. For example, changes in the economy that 25 to fewer job opportunities for youth and rising unemployment 26 make gainful employment increasingly difficult to obtain. The resulting discontent may in27 lead more youths into criminal behavior. Families have also 28 changes these years. More families consist of oneparent households or two working parents; 29, children are likely to have less supervision at home, 30 was common in the traditional family 31. This lack of parental supervision is thought to be an influence on juvenile crime rates. Other 32 causes of offensive acts include frustration or failure in school, the increased 33 of drugs and alcohol, and the growing 34 of child abuse and child neglect. All these conditions tend to increase the probability of a child committing a criminal act, 35 a direct causal relationship has not yet been established.

A:point B:lead C:come D:amount

Many theories concerning the causes of juvenile delinquency crimes committed by young people focus either on the individual or on society as the major contributing influence. Theories 16 on the individual suggest that children engage in criminal behavior 17 they were not sufficiently penalized for previous misdeeds or that they have learned criminal behavior through 18 with others. Theories focusing on the role of society suggest that children commit crimes in 19 to their failure to rise above their socioeconomic status 20 as a rejection of middleclass values. Most theories of juvenile delinquency have focused on children from disadvantaged families, 21 the fact that children from wealthy homes also commit crimes. The latter may commit crimes 22 lack of adequate parental control. All theories, however, are tentative and are 23 to criticism. Changes in the social structure may indirectly 24 juvenile crime rates. For example, changes in the economy that 25 to fewer job opportunities for youth and rising unemployment 26 make gainful employment increasingly difficult to obtain. The resulting discontent may in 27 lead more youths into criminal behavior. Families have also 28 changes these years. More families consist of oneparent households or two working parents; 29, children are likely to have less supervision at home, 30 was common in the traditional family 31. This lack of parental supervision is thought to be an influence on juvenile crime rates. Other 32 causes of offensive acts include frustration or failure in school, the increased 33 of drugs and alcohol, and the growing 34 of child abuse and child neglect. All these conditions tend to increase the probability of a child committing a criminal act, 35 a direct causal relationship has not yet been established.

A:point B:lead C:come D:amount

Many theories concerning the causes of juvenile delinquency crimes committed by young people focus either on the individual or on society as the major contributing influence. Theories 16 on the individual suggest that children engage in criminal behavior 17 they were not sufficiently penalized for previous misdeeds or that they have learned criminal behavior through 18 with others. Theories focusing on the role of society suggest that children commit crimes in 19 to their failure to rise above their socioeconomic status 20 as a rejection of middleclass values. Most theories of juvenile delinquency have focused on children from disadvantaged families, 21 the fact that children from wealthy homes also commit crimes. The latter may commit crimes 22 lack of adequate parental control. All theories, however, are tentative and are 23 to criticism. Changes in the social structure may indirectly 24 juvenile crime rates. For example, changes in the economy that 25 to fewer job opportunities for youth and rising unemployment 26 make gainful employment increasingly difficult to obtain. The resulting discontent may in 27 lead more youths into criminal behavior. Families have also 28 changes these years. More families consist of oneparent households or two working parents; 29, children are likely to have less supervision at home, 30 was common in the traditional family 31. This lack of parental supervision is thought to be an influence on juvenile crime rates. Other 32 causes of offensive acts include frustration or failure in school, the increased 33 of drugs and alcohol, and the growing 34 of child abuse and child neglect. All these conditions tend to increase the probability of a child committing a criminal act, 35 a direct causal relationship has not yet been established.

A:point B:lead C:come D:amount