托福-50及答案解析.doc

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1、托福-50 及答案解析(总分：120.00，做题时间：90 分钟)一、READING(总题数：0，分数：0.00)二、Set 1(总题数：1，分数：14.00)POPULATION ECOLOGY1 Population ecology is the science that measures changes in population size and composition and identifies the causes of these fluctuations. Population ecology is not concerned solely with the human po

2、pulation. In ecological terms, a population consists of the individuals of one species that simultaneously occupy the same general area, rely on the same resources, and are affected by similar environmental factors. The characteristics of a population are shaped by its size and by the interactions a

3、mong individuals and between individuals and their environment. 2 Population size is a balance between factors that increase numbers and factors that decrease numbers. Some factors that increase populations are favorable light and temperature, adequate food supply, suitable habitat, ability to compe

4、te for resources, and ability to adapt to environmental change. Factors that decrease populations are insufficient or excessive light and temperature, inadequate food supply, unsuitable or destroyed habitat, too many competitors for resources, and inability to adapt to environmental change. 3 An imp

5、ortant characteristic of any population is its density. Population density is the number of individuals per unit, such as the number of maple trees per square kilometer in a county. Ecologists can rarely determine population size by actually counting all individuals within geographical boundaries. I

6、nstead, they often use a variety of sampling techniques to estimate densities and total population sizes. In some cases, they estimate population size through indirect indicators, such as the number of nests or burrows, or signs such as tracks or droppings. 4 Another important population characteris

7、tic, dispersion, is the pattern of spacing among individuals within the population“s geographical boundaries. Various species are distributed in their habitats in different ways to take better advantage of food supplies and shelter, and to avoid predators or find prey. Within a population“s range, d

8、ensities may vary greatly because not all areas provide equally suitable habitat, and also because individuals space themselves in relation to other members of the population. 5 Individuals may be spread across the environment in three possible dispersion patterns: clumped, uniform, or random. A clu

9、mped dispersion means that individuals are gathered in patches throughout their habitat. Clumping often results from the irregular distribution of resources needed for survival and reproduction. For example, fallen trees keep the forest floor moist, and many forest insects are clumped under logs whe

10、re the humidity is to their liking. Clumping may also be associated with mating, safety, or other social behavior. Crane flies, for example, swarm in great numbers, a behavior that increases mating chances, and some fish swim in large schools so they are less likely to be eaten by predators. 6 A uni

11、form dispersion means that the members of a population are evenly spread throughout their habitat. Uniform spacing occurs when individuals are able to survive anywhere in their habitat, but must also interact directly with other individuals in the population. For example, regular spacing of plants m

12、ay result from shading and competition for water. In animal populations, uniform distribution is usually caused by competition for some resource or by social interactions that set up individual territories for feeding, breeding, or resting. Individuals use the resources found immediately around them

13、 and spread out to use all of the available resources. 7 While clumping and uniform patterns occur where there are direct interactions among individuals, random spacing may be seen where neither strong attraction nor strong repulsion exists among individuals in a population. In random dispersion, in

14、dividuals are spread throughout their habitat in an unpredictable, irregular manner. Each individual“s location is independent of the locations of other individuals. In tropical rain forests, trees of the same species can be randomly dispersed. Overall, random patterns are rare in nature, with most

15、populations showing a tendency toward either clumped or uniform distribution. Populations change in size, structure, and distribution as they respond to changes in environmental conditions. Four main variablesbirths, deaths, immigration, and emigrationdetermine the rate of change in the size of the

16、population over time. A change in the birth rate or death rate is the major way that most populations respond to changes in resource availability. Members of some animal species can avoid or reduce the effects of environmental stress by emigrating from one area and immigrating to another with more f

17、avorable environmental conditions, thus altering the population“s dispersion.（分数：14.00）(1).Which sentence below best expresses the essential information in the highlighted sentence in paragraph 1? Incorrect choices change the meaning in important ways or leave out essential information.（分数：1.00）A.An

18、y species of life can be studied in population ecology.B.Population ecologists care about the future of humanity.C.The growth of the human population is a major concern.D.Population ecology does not consider humans worthy of study.(2).According to the passage, which factor might cause the population

19、 of a species to decrease in size?（分数：1.00）A.A favorable amount of light and waterB.An ability to hide from or defend against predatorsC.A large number of other species competing for foodD.A greater number of births than deaths(3).Which of the following is an indirect indicator of a population“s den

20、sity?（分数：1.00）A.The distribution of food in a given areaB.The number of nests in a given areaC.The number of births in a given period of timeD.The number of individuals counted in a given area(4).The distribution pattern of individuals within a population“s geographical boundaries is known as（分数：1.0

21、0）A.population ecologyB.population densityC.population changeD.population dispersion(5).The word range in paragraph 4 is closest in meaning to（分数：1.00）A.territoryB.controlC.historyD.shelter(6).The word patches in paragraph 5 is closest in meaning to（分数：1.00）A.dark placesB.family groupsC.warm spotsD.

22、small areas(7).All of the following are given as reasons for clumping EXCEPT（分数：1.00）A.uneven resource distributionB.territorial disputesC.mating behaviorD.safety from predators(8).The phrase set up in paragraph 6 is closest in meaning to（分数：1.00）A.forbidB.establishC.increaseD.conceal(9).Which of th

23、e following situations would be most likely to result in a uniform dispersion pattern?（分数：1.00）A.Birds compete for a place to build their nests.B.Fish swim in large schools to avoid predators.C.Whales develop strong bonds among relatives.D.Elephants form a circle to protect their young.(10).Random s

24、pacing in a population differs from other dispersion patterns in that it（分数：1.00）A.occurs when individuals must compete for resourcesB.can be found in nearly every type of habitatC.does not depend on interactions among membersD.is not related to environmental conditions(11).The word repulsion in par

25、agraph 7 is closest in meaning to（分数：1.00）A.avoidanceB.similarityC.innovationD.complexity(12).Why does the author mention immigration and emigration in paragraph 8?（分数：1.00）A.To identify factors affecting population dispersionB.To give examples of territorial behavior in animalsC.To show that popula

26、tions balance themselves over timeD.To explain why animal populations are uniformly dispersed(13).Look at the four squares, A, B, C, and D, which indicate where the following sentence could be added to the passage. Where would the sentence best fit? For example, they might estimate the number of bla

27、ck bears in a national park by counting Individuals in a few sample plots representative of the whole park. An important characteristic of any population is its density. A Population density is the number of individuals per unit, such as the number of maple trees per square kilometer in a county. B

28、Ecologists Can rarely determine population size by actually counting all individuals within geographical boundaries. C Instead, they often use a variety of sampling techniques to estimate densities and total population sizes. D In some cases, they estimate population size through indirect indicators

29、, such as the number of nests or burrows, or signs such as tracks or droppings.（分数：1.00）(14).An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences

30、do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Population ecology is the science of populations and the factors that influence them. Answer ChoicesA. A population is affected by its siz

31、e, density, and dispersion, as well as the relationships among its members. B. Dispersion of individuals in a population may be clumped, uniform, or random, as determined by environmental and social factors. C. Clumping is related to safety; for example, fish swim in large schools to reduce their ch

32、ances of being eaten by predators. D. Most populations have either clumped or uniform distribution because random spacing is rare in nature. E. Populations change in density and dispersion as they respond to changes in environmental conditions and resource availability. F. If a population has an abu

33、ndant food supply and a suitable habitat, it will not undergo evolutionary change.（分数：1.00）三、Set 2(总题数：1，分数：13.00)THE NORSE IN NORTH AMERICA1 The Norse made the first documented European voyages to North America, and there is evidence of these visits in the medieval sagas, a collection of stories th

34、at tell the history of the Icelandic people. The Icelandic sagas relate how the Norse captain Leif Eriksson and his brother sailed from Norway to Greenland and landed to the west of Greenland. The sagas also describe Thorfinn Karlsefni“s attempt to colonize a place called Vinland. Although some hist

35、orians doubt their accuracy, the sagas are a source of intriguing details, and until the twentieth century they were the primary source of information about the Norse exploration of North America. 2 In the tenth century, Norsemen ventured from their homeland in Scandinavia to build settlements on th

36、e Greenland coast. A trader named Bjarni Herjolfsson sailed for Greenland in the summer of 985 or 986. Driven off course by a storm, he subsequently discovered a coastline he described as wooded and hilly. Although Bjarni did not go ashore, this discovery made him the first European to set eyes on t

37、he continent of North America. Bjarni“s account of his voyage encouraged another Norseman, Leif Eriksson, to undertake a southward voyage, starting around the year 1000, with a crew of 35 men. 3 According to the sagas, Leif Eriksson and his men first landed in a place they called Stoneland, which wa

38、s probably the rocky, barren Labrador coast of North America. The party finally landed in a place they called Vinland, where they spent a winter in rough huts in a seemingly frost-free land of abundant vines and wild grapes. They established the first European colony in North America at Vinland, the

39、 precise location of which remains a subject of scholarly dispute to this day. The Norsemen returned home in the spring, abandoning the rude settlement that, a few years later, would serve as home base for Thorfinn Karlsefni of Greenland. 4 Around 1004, the expedition led by Thorfinn set off southwa

40、rd, evidently with a longer stay in mind, as women and cattle accompanied the sailors in three ships. The party of 160 made several landings and eventually found a place with vines, where they settled at a site up a river that widened into a lake. They passed two years at Vinland, exploring the coas

41、t, building huts, fishing, and gathering native foods. During that time, Thorfinn“s wife gave birth to a boy, the first child of European descent born in the Americas. The settlers had several hostile encounters with the local aboriginal tribes, whom they called “skrelings.“ In one fight with a nati

42、ve group, Thorfinn and another Norseman were killed. The continuing prospect of attack and growing dissension in the settlement may have thwarted the Norse attempt at colonization. For some reason, they departed their settlement; however, Greenlanders continued to make occasional visits there in lat

43、er years, using it as a fishing camp. Historians have made many attempts to identify the lands visited by Thorfinn and to discover his campsites, but no single theory has won general acceptance. 5 In the twentieth century, the Icelandic sagas served as inspiration for Norwegian explorer and writer H

44、elge Ingstad, who in the early 1960s traveled the coasts of eastern North America searching for evidence of Vinland. Encouraged by an alternative interpretation of “vin“ as meaning “meadow“ rather than vine or wine, Ingstad discovered a grassy site on the northern tip of Newfoundland that local peop

45、le had believed was an aboriginal site haunted by ancient ghosts. Ingstad excavated the remains of eight sod huts, together with bronze and stone tools of Norse origin. He concluded that the grassland called L“Anse aux Meadows was, if not the Vinland of the sagas, then certainly a Norse settlement o

46、f some kind. 6 Huddled for protection from the wind, the cluster of sod-built structures at L“Anse aux Meadows was no temporary camp. Sod walls and sod roofs built over a timber frame indicated dwellings that were substantial enough for permanent occupation. The eight huts included three long, narro

47、w buildings with features similar to those found in Norse structures in Greenland and Iceland. The largest of these measured 29 by 15 meters and consisted of several rooms. Smaller buildings, probably used for storage and workshops, included a forge that used iron ore extracted from peat bogsevidenc

48、e of the first iron working in North America. Also among the artifacts were a spindle and a bone knitting needle, suggesting the presence of women in the settlement. Glossary: sod: soil held together by the roots of grass（分数：13.00）(1).What can be inferred from paragraph 1 about the Icelandic sagas?（

49、分数：1.00）A.They were not known outside of Iceland.B.They were originally told in the form of songs.C.They may not be historically accurate.D.They inspired Leif Eriksson to explore Vinland.(2).The phrase set eyes on in paragraph 2 is close, in meaning to（分数：1.00）A.colonizeB.observeC.imagineD.explore(3).According to the passage, who was probably the first European to step on the continent of North America?（分数：1.00）A.Bjarni HerjolfssonB.Leif ErikssonC.Thorfinn KarlsefniD.Helge Ingstad(4).The word abundant in paragraph 3 is closest in meaning to（分数：1.00）A.numerousB.tropicalC.familiarD.pois