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2008年12月大学英语四级模拟冲刺卷6

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Passage Two

Questions 62 to 66 are based on the following passage.

Rising global carbon dioxide levels tied to global warming may not be as crucial in determining the position of plant munities as other, localized climate changes.

"Nobody really knows what the increases in carbon dioxide are going to entail in terms of future changes in vegetation types," said Mark Brenner, a University of Florida assistant professor of paleolimnology, the study of ancient lakes. "It looks like climate changes in different areas may be more important than carbon dioxide, at least carbon dioxide by itself," he said.

Brenner’s research team based their conclusions on an analysis of sediment from two lake bottoms, one in northern Mexico and one in northern Guatemala. The researchers used new techniques that allowed them to analyze only the remains of land plants, specifically their leaf waxes. By measuring the position of the leaf waxes, the researchers were able to distinguish two broad categories of plants living in these areas -- so-called C3 and C4 plants, which he different photosynthetic(光合作用) processes. Many C4 plants are tropical grasses, while most tropical trees are C3 plants. The researchers analyzed sediments(沉积物) deposited over the last 27,000 years, from the last ice age to the current geological period. Over this period, there was a worldwide, relatively uniform increase in atmospheric carbon dioxide concentrations.

Brenner said that if carbon dioxide played the major role in determining plant position, one would assume that analysis of the sediments would reveal very similar changes in relative abundance of C3 and C4 plants in the two places over the study period. But, in fact, the researchers found that trends in the two types of plants were different at the two locations. The changes were related not with carbon dioxide levels, but with shifts in rainfall. "The result appears to be that climate factors, especially moisture ailability, determine whether C4 or C3 plants dominate in an area, not carbon dioxide," Brenner said.

Many scientists believe global warming will cause major variation in local climates worldwide, with some wet areas being dry and dry areas being wet. If that happens, it could he more impact on relative C3 versus C4 plant distribution than the rising carbon dioxide levels.

62. What can be inferred in the first paragraph?

A) Climate changes are more important to the position of plant munities than rising global carbon dioxide.

B) Localized climate shifts may not be as crucial as carbon dioxide.

C) Nobody knows which one is important.

D) Carbon dioxide levels is crucial to the global warming.

63. What is Mark Brenner?

A) He studies co-author’s opinion.

B) He is assisting the University of Florida.

C) He is an expert in the field of ancient lakes.

D) His research team posed of six geologists and geographers.

64. According to the third paragraph, which one is NOT true?

A) Tropical grasses are usually C4 plants.

B) C3 and C4 plants used to live in northern Mexico and Guatemala.

C) C3 and C4 plants don’t he the same processes.

D) Tropical trees are all C3 plants.

65. Why, in the 4th paragraph, the researchers found that trends in C3 and C4 plants were different at the two locations?

A) The assumption that carbon dioxide played the major role is wrong.

B) The carbon dioxide played an important role.

C) The moisture ailability was different.

D) The carbon dioxide level was different.

66. What’s the main idea of the passage?

A) Climates factors determine the plant distribution and position of plant munities.

B) Global warming will cause major variation.

C) How has Brenner’s research team proved a truth.

D) C3 and C4 plants are important plants in determining the position of plant munities.

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