Bracken, a poisonous weed, is spreading and damagi

单项选择题

Questions 4—5 are based on the following:
Any person who drops out of high school will be unemployed unless he or she finds a low-paying job or has relatives with good business connections.

Bracken, a poisonous weed, is spreading and damaging much pastureland in the Northern Hemisphere. One potentially inexpensive and self-sustaining countermeasure is to introduce natural enemies of the plant; therefore, some scientists have proposed to control bracken by a release of brackeneating moths native to the Southern Hemisphere into brackeninfested areas in the Northern Hemisphere. If the scientists’ proposal for controlling bracken is adopted, which of the following is a necessary condition for its success

A. That bracken in the Northern Hemisphere grows in approximately the same climates and soil conditions in which it grows in the Southern Hemisphere.
B. That the released moths will feed on weeds other than bracken that are native to the Northern Hemisphere.
C. That the livestock that will return to pastures now lost to bracken will develop immunities to the diseases caused by bracken.
D. That the released moths will survive in sufficient numbers to build a population large enough to reduce bracken and retard its growth.
E. That traditional methods of control, such as burning, cutting, and chemical spraying, will not become less expensive or labor-intensive than they are now.

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