Climatic conditions are delicately adjusted to composition of the Earth's atmosphere. If there were a change in the atmosphere ― for example, in the relative amounts of atmospheric gases ― the climate would probably change also. A slight increase in water vapor (蒸汽) , for example, would increase the heat-keeping capacity of the atmosphere and would lead to a rise in global temperatures.

The level of carbon dioxide, CO₂, in the atmosphere has an important effect on climatic change. Most of the Earth's incoming energy is short-wavelength radiation (短波辐射) , which tends to pass through atmospheric CO₂ easily. The Earth, however, reradiates much of the received energy as long-wavelength radiation (长波辐射), which CO₂ absorbs and then goes toward the Earth. This phenomenon, known as the greenhouse effect, can result in an increase in the surface temperature of a planet. An extreme example of the effect is shown by Venus, a planet covered by heavy clouds composed mostly of CO₂, whose surface temperatures have been measured at 430~C.If the CO₂ content of the atmosphere is reduced, the temperature falls. According to one respectable theory, if the atmospheric CO₂ concentration were halved, the Earth would become completely covered with ice.  Another equally respectable theory, however, states that a halving of the CO₂ concentration would lead only to a reduction in global temperatures of 3℃.

If, because of an increase in forest fires or volcanic activity, the CO₂ content of the atmosphere increased, a warmer climate would be produced. Plant growth, which relied on both the warmth and the availability of CO₂, would probably increase. As a consequence, plant would use more and more CO₂.  Eventually CO₂ levels would decrease and the climate, in turn, would become cooler. With reduced temperatures many plants would die; CO₂ would thus be returned to the atmosphere and gradually the temperature would rise again. Thus, if this process occurred, there might be a long-term oscillation (波动) in the amount of CO₂ present in the atmosphere, with regular temperature increase and decrease.

Some climatologists argue that the burning of fossil fuels has raised the level of CO₂ in the atmosphere and has caused a global temperature increase of at least 1℃. But a supposed global temperature rise of 1 ℃ may in reality be only several regional temperature increases, restricted to areas where there are many meteorological (气象的) stations. Other areas, for example, the Southern Hemisphere oceanic zone, may be experiencing an equal temperature decrease that is unrecognized because of the shortage of meteorological recording stations.

51. Which of the following questions does the information supplied in this passage try to answer?

A. What are the steps in the process that takes place as CO₂ absorbs long-wavelength radiation?

B. How might our understanding of the greenhouse effect be improved if the burning of fossil  were decreased?

C. What might cause a series of regular increases and decreases in the amount of CO₂ in the atmosphere?

D. Why are there fewer meteorological recording stations in the Southern Hemisphere oceanic zone than atmosphere?

52. The author is primarily concerned with ________.

A. explaining the effects that the burning of fossil fuels might have on climate

B. explaining the effects of CO₂ on atmospheric radiation

C. discussing effects that changes in the CO₂ level in the atmosphere might have on climate

D. challenging hypotheses (假说)  about the effects of water vapor and CO₂ on climate

53. The passage suggests that a large decrease in the amount of CO₂ in the atmosphere would result in _________.

A. at least a slight decrease in global temperatures

B. at the most a slight increase in short-wavelength radiation reaching the Earth

C. a slight long-term increase in global temperatures

D. a slight short-term increase in atmosphere water vapor content

54. The author refers to Venus primarily in order to _______.

A. show that the greenhouse effect works on other planets but not on Earth

B. show the degree to which Earth's atmosphere differs from that of Venus

C. support the opinion that as water vapor increases, the amount of CO₂ increases

D. support the argument that the CO₂ level in the atmosphere has a significant effect on climate

55. The discussion of climate in the passage suggests that _________.

A. climate is perfectly stable

B. we are unable at present to measure global temperature changes precisely.

C. the only cause of regional climatic fluctuations (波动) is the change in CO₂ 1evel in the atmosphere

D. slight regional temperature changes can be considered an abnormal feature (特征) of the environment

Farming was once the chief way of life in nearly every country. People cannot live without food, and nearly all their food comes from crops and animals raised on farms. Not many people farm for a living any more, but farming remains the most important work in the world.

    Before the nineteenth century, the typical American family lived on a small farm. They raised pigs, cattle, sheep, chickens, and planted corn, fruits, garden vegetables, and wheat. Everyone worked long and hard, but the results were often poor. Families barely harvested enough food for themselves. This situation began to change during the last half of the 1800's and it changed remarkably(明显地) in the next century.

Scientific methods and labor-saving machinery have made farming increasingly productive. The development of improved plant varieties and fertilizers has helped double and even triple the production of some major crops. More scientific animal care and animal raising have helped increase the amount of meat and products that animals produce. At the same time, the use of tractors and other modern farm equipment has sharply reduced the need for farm labor.

As farming has become less important as a way of life in the United States, it has become more important as a source which offers materials to industry. Today's successful farmers are experts not just in agriculture but also in accounting, marketing, and finance. Farms that are not run in a similar way have great difficulty surviving.

We are told in the first paragraph about ____.

A. the history of farming                       B. the importance of farming  

C. the development of farming                  D. the changes of farming

Which of the following describes American farming before the 19^th century?

A. Farmers didn’t work hard on their farms.  

B. Farmers used tractors to help them.

C. Farming was the main way of living for most people.

D. Farmers could supply materials to industry.

Crop production has increased greatly now thanks to all the following EXCEPT __.

   A. scientific methods                 B. labor- saving machinery    

C. farmers’ hard work                 D. chemical fertilizers

The passage mainly talks about_____.

   A. the advantages of American farming              B. the changes in American farming 

C. the situation of American farms                  D. how rapidly farming has developed

Farming was once the chief way of life in nearly every country. People cannot live without food, and nearly all their food comes from crops and animals raised on farms. Not many people farm for a living any more, but farming remains the most important work in the world.
Before the nineteenth century, the typical American family lived on a small farm. They raised pigs, cattle, sheep, chickens, and planted corn, fruits, garden vegetables, and wheat. Everyone worked long and hard, but the results were often poor. Families barely harvested enough food for themselves. This situation began to change during the last half of the 1800's and it changed remarkably(明显地) in the next century.
Scientific methods and labor-saving machinery have made farming increasingly productive. The development of improved plant varieties and fertilizers has helped double and even triple the production of some major crops. More scientific animal care and animal raising have helped increase the amount of meat and products that animals produce. At the same time, the use of tractors and other modern farm equipment has sharply reduced the need for farm labor.
As farming has become less important as a way of life in the United States, it has become more important as a source which offers materials to industry. Today's successful farmers are experts not just in agriculture but also in accounting, marketing, and finance. Farms that are not run in a similar way have great difficulty surviving.
1. We are told in the first paragraph about ____.
A. the history of farming                       B. the importance of farming  
C. the development of farming                  D. the changes of farming
2. Which of the following describes American farming before the 19th century?
A. Farmers didn’t work hard on their farms.  
B. Farmers used tractors to help them.
C. Farming was the main way of living for most people.
D. Farmers could supply materials to industry.
3. Crop production has increased greatly now thanks to all the following EXCEPT __.
A. scientific methods                 B. labor- saving machinery    
C. farmers’ hard work                 D. chemical fertilizers
4. Which of the following is NOT mentioned in the passage?
A. Traditional American farming.            B. The increase in the number of crops in the US.
C. Modern farming methods.                D. American farmers’ hard work.
5. The passage mainly talks about_____.
A. the advantages of American farming      B. the changes in American farming 
C. the situation of American farms          D. how rapidly farming has developed

Farming was once the chief way of life in nearly every country. People cannot live without food, and nearly all their food comes from crops and animals raised on farms. Not many people farm for a living any more, but farming remains the most important work in the world.
Before the nineteenth century, the typical American family lived on a small farm. They raised pigs, cattle, sheep, chickens, and planted corn, fruits, garden vegetables, and wheat. Everyone worked long and hard, but the results were often poor. Families barely harvested enough food for themselves. This situation began to change during the last half of the 1800's and it changed remarkably(明显地) in the next century.
Scientific methods and labor-saving machinery have made farming increasingly productive. The development of improved plant varieties and fertilizers has helped double and even triple the production of some major crops. More scientific animal care and animal raising have helped increase the amount of meat and products that animals produce. At the same time, the use of tractors and other modern farm equipment has sharply reduced the need for farm labor.
As farming has become less important as a way of life in the United States, it has become more important as a source which offers materials to industry. Today's successful farmers are experts not just in agriculture but also in accounting, marketing, and finance. Farms that are not run in a similar way have great difficulty surviving.
【小题1】 We are told in the first paragraph about ____.

   Farming was once the chief way of life in nearly every country. People cannot live without food, and nearly all their food comes from crops and animals raised on farms. Not many people farm for a living any more, but farming remains the most important work in the world.

    Before the nineteenth century, the typical American family lived on a small farm. They raised pigs, cattle, sheep, chickens, and planted corn, fruits, garden vegetables, and wheat. Everyone worked long and hard, but the results were often poor. Families barely harvested enough food for themselves. This situation began to change during the last half of the 1800's and it changed remarkably(明显地) in the next century.

Scientific methods and labor-saving machinery have made farming increasingly productive. The development of improved plant varieties and fertilizers has helped double and even triple the production of some major crops. More scientific animal care and animal raising have helped increase the amount of meat and products that animals produce. At the same time, the use of tractors and other modern farm equipment has sharply reduced the need for farm labor.

As farming has become less important as a way of life in the United States, it has become more important as a source which offers materials to industry. Today's successful farmers are experts not just in agriculture but also in accounting, marketing, and finance. Farms that are not run in a similar way have great difficulty surviving.

1. We are told in the first paragraph about ____.

A. the history of farming                       B. the importance of farming  

C. the development of farming                  D. the changes of farming

2. Which of the following describes American farming before the 19th century?

A. Farmers didn’t work hard on their farms.  

B. Farmers used tractors to help them.

C. Farming was the main way of living for most people.

D. Farmers could supply materials to industry.

3. Crop production has increased greatly now thanks to all the following EXCEPT __.

   A. scientific methods                 B. labor- saving machinery    

C. farmers’ hard work                 D. chemical fertilizers

4. Which of the following is NOT mentioned in the passage?

   A. Traditional American farming.            B. The increase in the number of crops in the US.

   C. Modern farming methods.                D. American farmers’ hard work.

5. The passage mainly talks about_____.

   A. the advantages of American farming      B. the changes in American farming 

C. the situation of American farms          D. how rapidly farming has developed