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Bracken fern has been spreading from its woodland strongholds for centuries, but the rate of encroachment into open countryside has lately increased alarmingly through-out northern and western Britain. A tough competitor, bracken reduces the value of grazing land by crowding out other vegetation. The fern is itself poisonous to livestock, and also encourages proliferation of sheep ticks, which not only attack sheep but also transmit diseases. No less impor- tant to some people are bracken's effects on threatened habitats and on the use of uplands for recreational pur- poses, even though many appreciate its beauty. Biological controls may be the only economic solution. One potentially cheap and self-sustaining method of halting the spread of bracken is to introduce natural enemies of the plant. Initially unrestrained by predators of their own, foreign predators are likely to be able to multiply rapidly and overwhelm intended targets. Because bracken occurs throughout the world, there is plenty of scope for this approach. Two candidates, both moths from the Southern Hemisphere, are now being studied. Of course, biological control agents can safely be released only if it can be verified that they feed solely on the target weed. The screening tests have so far been fraught with difficulties. The first large shipment of moths succumbed to a disease. Growing enough bracken indoors is difficult, and the moths do not readily exploit cut stems. These are common problems with rearing insects for bio- logical control.

Other problems can be foreseen. Policymakers need to consider many factors and opinions such as the cost of control compared to existing methods, and the impact of the clearance of bracken on the landscape, wildlife, and vegetation. In fact, scientists already have much of the information needed to assess the impact of biological control of bracken, but it is spread among many individuals, organizations, and government bodies. The potential gains for the environment are likely to outweigh the losses because few plants, insects, mammals, and birds live associated only with bracken, and many would benefit from a return of other vegetation or from a more diverse mosaic of habitats. But legal consequences of attempts at biological control present a potential minefield. For exam- ple, many rural tenants still have the right of “estoyers” the right to cut bracken as bedding for livestock and uses. What would happen if they were deprived of these rights? Once a biological control agent is released, it is difficult to control its speed. What consideration is due landowners who do not want to control bracken? Accord- ing to law, the release of the biological control agents must be authorized by the secretary of state for the environment. But Britain lacks the legal and administrative 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, careful assessment of the consequences is required.
C.Environmentalists are hoping that laboratory technicians will find a way to raise large numbers 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, the government 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 encroachment EXCEPT:
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 the general 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 biological control agent a predator that is foreign to the targeted environment for which of the following 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 the biological 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 can best be described as
A.rapidly expanding
B.the subject of controversy
C.well established
D.circumscribed by numerous predators
E.a significant nutrient source

参考答案

正确答案:B A D E E C C