GRE阅读模拟在线练习附答案二百四十七

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　　Bracken fern has been spreading from its woodlandstrongholds for centuries, but the rate ofencroachment into open countryside has latelyincreased alarmingly through-out northern andwestern Britain. A tough competitor, bracken reducesthe value of grazing land by crowding out othervegetation. The fern is itself poisonous tolivestock, and also encourages proliferation ofsheep ticks, which not only attack sheep but also transmit diseases. No less impor- tant tosome people are bracken's effects on threatened habitats and on the use of uplands forrecreational pur- poses, even though many appreciate its beauty. Biological controls may bethe only economic solution. One potentially cheap and self-sustaining method of halting thespread of bracken is to introduce natural enemies of the plant. Initially unrestrained bypredators of their own, foreign predators are likely to be able to multiply rapidly andoverwhelm intended targets. Because bracken occurs throughout the world, there is plenty ofscope for this approach. Two candidates, both moths from the Southern Hemisphere, are nowbeing studied. Of course, biological control agents can safely be released only if it can beverified that they feed solely on the target weed. The screening tests have so far beenfraught with difficulties. The first large shipment of moths succumbed to a disease. Growingenough bracken indoors is difficult, and the moths do not readily exploit cut stems. These arecommon problems with rearing insects for bio- logical control.

　　Other problems can be foreseen. Policymakers need to consider many factors and opinions suchas the cost of control compared to existing methods, and the impact of the clearance ofbracken on the landscape, wildlife, and vegetation. In fact, scientists already have much of theinformation needed to assess the impact of biological control of bracken, but it is spreadamong many individuals, organizations, and government bodies. The potential gains for theenvironment are likely to outweigh the losses because few plants, insects, mammals, andbirds live associated only with bracken, and many would benefit from a return of othervegetation or from a more diverse mosaic of habitats. But legal consequences of attempts atbiological control present a potential minefield. For exam- ple, many rural tenants still havethe right of “estoyers” the right to cut bracken as bedding for livestock and uses. What wouldhappen if they were deprived of these rights? Once a biological control agent is released, it isdifficult to control its speed. What consideration is due landowners who do not want tocontrol bracken? Accord- ing to law, the release of the biological control agents must beauthorized by the secretary of state for the environment. But Britain lacks the legal andadministrative machinery to assemble evidence for and against release.

　　Which of the following best states the main idea of the passage?

　　A.Studies suggest that biologicalcontrol of bracken will not be technically feasible.

　　B.Although biological control appears to be the best solution to bracken infestation, carefulassessment of the consequences is required.

　　C.Environmentalists are hoping that laboratory technicians will find a way to raise largenumbers of moths in captivity.

　　D.Bracken is currently the best solution to the proliferation of nonnative moth species.

　　E.Even after researchers discover the most economical method of pest control, thegovernment has no authority to implement a control program.

　　According to the passage, which of the following can be inferred about sheep ticks?

　　A.They increase where bracken spreads.

　　B.They are dangerous only to sheep.

　　C.They are especially adapted to woodland.

　　D.They have no natural enemies.

　　E.They cause disease among bracken.

　　The author cites all of the following as disadvantages of bracken encroachmentEXCEPT:

　　A.Bracken is poisonous to farm animals.

　　B.Bracken inhibits the growth of valuable vegetation.

　　C.Bracken indirectly helps spread certain diseases.

　　D.Bracken is aesthetically objectionable.

　　E.Bracken disturbs habitats that some people would like to protect.

　　The final paragraph can best be described as

　　A.a summation of arguments presented in previous paragraphs

　　B.the elimination of competing arguments to strengthen a single remaining conclusion

　　C.an enumeration of advantages to biological control

　　D.an expansion of the discussion from the particular example of bracken control to thegeneral problem of government regulation

　　E.an overview of the variety of factors requiring further assessment

　　It can be inferred from the passage that it is advantageous to choose as the biologicalcontrol agent a predator that is foreign to the targeted environment for which of thefollowing reasons?

　　A.Conservation groups prefer not to favor one native species over another.

　　B.All local predators have already been overwhelmed by the target species.

　　C.Local predators cannot be effectively screened since they already exist in the wild.

　　D.There is little risk of an artificially introduced foreign predator multiplying out of control.

　　E.Native predator species are generally limited by their own predators..

　　It can be inferred from the passage that the screening tests performed on thebiological control agent are designed primarily to determine

　　A.its effectiveness in eliminating the target species

　　B.the response of local residents to its introduction

　　C.the risk it poses to species other than the target

　　D.its resistance to the stress of shipment

　　E.the likelihood of its survival indoors

　　As it is discussed in the passage, the place of bracken within the forest habitat canbest be described as

　　A.rapidly expanding

　　B.the subject of controversy

　　C.well established

　　D.circumscribed by numerous predators

　　E.a significant nutrient source

　　正确答案:B A D E E C C

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