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2011-09-28 16:18:00   来源：作文地带整理    双击单词自动翻译

　　(65) are quickly suppressed by viral

　　epidemics.

　　Further research has shown that cater-

　　pillars become virtually immune to the

　　wilt virus as the trees on which they

　　(70) feed respond to increasing defoliation.

　　The trees own defenses raise the

　　threshold of caterpillar vulnerability

　　to the disease, allowing populations

　　to grow denser without becoming

　　(75) more susceptible to infection. For

　　these reasons, the benefits to the

　　caterpillars of ingesting phenols

　　appear to outweigh die costs. Given

　　the presence of the virus, the trees

　　(80) defensive tactic apparently has

　　backfired.

　　Which of the following statements best

　　expresses the main point of the passage?

　　Recurring outbreaks of infestation by

　　gypsy moth caterpillars have had a

　　devastating impact on trees in the

　　northeastern United States.

　　A mechanism used by trees to combat

　　the threat from gypsy moth caterpillars

　　has actually made some trees more

　　vulnerable to that threat.

　　Although deadly to gypsy moth cater-

　　pillars, wilt disease has failed to

　　significantly affect the population

　　density of the caterpillars.

　　The tree species with the highest levels

　　of phenols in their foliage are the most

　　successful in defending themselves

　　against gypsy moth caterpillars.

　　In their efforts to develop new,

　　methods for controlling gypsy moth

　　caterpillars, researchers have focused

　　on the effects of phenols in tree