A. poor farming

B. overpopulation

C. radical climate change

D. disappearance of rare plant species

答案：a

40. In order to prevent desertification,the author proposes ___.

A. making good use of international aids

B. developing a sustainableagricultural economy

C. gaining international support

D. converting agricultural land intoforest

答案：b

第三篇

OlderVolcanic Eruptions

Volcanoes were more destructive in ancient history, not because theywerebigger，but because the carbon dioxide(二氧化碳)theyreleased wiped out life withgreater ease.

Paul Wignall from the University of Leeds was investigating the linkbetweenvolcanic eruptions and mass extinctions. Not all volcanic eruptionskilled offlarge numbers of animals, but all the mass extinctions over the past300 millionyears coincided with huge formations of volcanic rock. To hissurprise, the olderthe massive volcanic eruptions were, the more damage theyseemed to do. Hecalculated the "killing efficiency" for thesevolcanoes by comparing theproportion of life they killed off with the volumeof lava (熔岩) that theyproduced. He found that sizefor size, older eruptions were at least 10 times aseffective at wiping outlife as their more recent rivals

The Permian (二叠纪)extinction, for example, which happened 250 million yearsago, is marked byfloods of volcanic rock in Siberia that cover an area roughlythe size ofwestern Europe. Those volcanoes are thought to have pumped out about10gigatonnes (十亿吨) of carbon as carbon dioxide. The globalwarming that followedwiped out 80 percent of all marine genera (种类)at the time, and it took 5 millionyears for the planet to recover. Yet 60million years ago, there was another hugeamount of volcanic activity andglobal warming but no mass extindtion. Someanimals did disappear but thingsreturned to normal within tens of thousands ofyears. "The most recentones hardly have an effect at all," Wignall says. Heignored theextinction which wiped out the dinosaurs (恐龙) 65million years ago,because many scientists believe it was primarily caused bythe impact of anasteroid (小行星). Hethinks that older volcanoes had more killing power becausemore recent lifeforms were better adapted to dealing with increased levels ofCO2.

Vincent Courtillot, director of the Paris Geophysical Institute inFrance,says that Wignall's idea is provocative. But he says it is incrediblyhard todo these sorts of calculations. He points out that the killing powerofvolcanic eruptions depends on how long they lasted. And it is impossibletotell whether the huge blasts lasted for thousands

or millions of years. He also adds that itis difficult to estimate how muchlava prehistoric volcanoes produced, and thatlava volume may not necessarilycorrespond to carbon dioxide emissions.

41.Older volcanic eruptions did moredamage than more recent ones because

A. older volcanoes were brighter.

B. carbon dioxide made the earth muchwarmer

C. older volcanoes were hotter

D. carbon dioxide killed off life moreeasily

答案：d

42. Wignall calculated the killing powerof those older volcanic eruptionsby

A. estimating how long they lasted

B. counting the dinosaurs they killed

C. comparing the proportion of lifekilled with the volume of lavaproduced

D. studying the chemical composition oflava

答案：c

43. When did dinosaurs become extinct?

A. 300 million years ago.

B. 250 million years ago

C. 65 million years ago

D. 60 million years ago

答案：c

44. It can be inferred from Paragraph 3that the cause of dinosaur extinctionis_______

A. a political issue.

B. self-evident.

C. quite certain

D. controversial

答案：d

45. What is the main thesis of thearticle?

A. Volcanic eruptions are not alwaysdeadly.

B. Carbon dioxide emissions often giverise to global warming.

C. Older volcanic eruptions are moredestructive

D. It is not easy to calculate the killingpower of a volcanic eruption

答案：c

补全短文

Researchers Discover Why Humans Began Walking Upright

Most of us walk and carry items in our hands every day. These are seeminglysimple activities that the majority of us don’t question. _____(46) The team ofresearchers from the U. S., England, Japan and Portugal investigated thebehavior of modern-day chimpanzees as they competed for food resources，in aneffort to understand what ecological settings would lead a large ape — one thatresembles the 6 million-year old ancestor we shared in common with livingchimpanzees — to walk on two legs.

"These chimpanzees provide a model of the ecological conditions under whichour earliest ancestors might have begun walking on two legs", said Dr.Richmond.

The research findings suggest that chimpanzees switch to moving on two limbsinstead of four in situations where they need to monopolize aresource._____(47)Over time, intense bursts of bipedal activity may have led toanatomical changes that in turn became the subject of natural selection wherecompetition for food or other resources was strong.

Two studies were conducted by the team in Guinea. The first study wasconducted by the team in Kyoto University's "outdoor laboratory" in a naturalclearing in Bossou Forest. _____(48)The chimpanzees' behavior was monitored inthree situations: when only oil palm nuts were available, when a small number ofcoula nuts were available，and when coula nuts were the majority availableresource.

When the rare coula nuts were available only in small numbers, thechimpanzees transported more at one time. Similarly, when coula nuts were themajority resource, the chimpanzees ignored the oil palm nuts altogether._____(49)

In such high-competition settings，the frequency of cases in which thechimpanzees started moving on two legs increased by a factor of four. Not onlywas it obvious that bipedal movement allowed them to carry more of this prec