Which of the following, if true, would most clearly demonstrate the operation of the trees` defensive mechanism as it is答案解析-雷哥GRE

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The defoliation of millions of acres of trees by massive infestations of gypsy moth caterpillars is a recurring phenomenon in the northeastern United States. In studying these outbreaks, scientists have discovered that affected trees fight back by releasing toxic chemicals, mainly phenols, into their foliage. These noxious substances limit caterpillars' growth and reduce the number of eggs that female moths lay. Phenols also make the eggs smaller, which reduces the growth of the following year's caterpillars. Because the number of eggs a female moth produces is directly related to her size, and because her size is determined entirely by her feeding success as a caterpillar, the trees' defensive mechanism has an impact on moth fecundity. The gypsy moth is also subject to attack by the nucleopolyhedrosis virus, or wilt disease, a particularly important killer of the caterpillars in outbreak years. Caterpillars contract wilt disease when they eat a leaf to which the virus, encased in a protein globule, has become attached. Once ingested by a caterpillar, the protein globule dissolves, releasing thousands of viruses, or virions, that after about two weeks multiply enough to fill the entire body cavity. When the caterpillar dies, the virions are released to the outside, encased in a new protein globule synthesized from the caterpillar's tissues and ready to be picked up by other caterpillars. Knowing that phenols, including tannins, often act by associating with and altering the activity of proteins, researchers focused on the effects on caterpillars of ingesting the virus and leaves together. They found that on tannin-rich oak leaves, the virus is considerably less effective at killing caterpillars than when it is on aspen leaves, which are lower in phenols. In general, the more concentrated the phenols in tree leaves, the less deadly the virus. Thus, while highly concentrated phenols in tree leaves reduce the caterpillar population by limiting the size of caterpillars and consequently, the size of the female's egg cluster, these same chemicals also help caterpillars survive by disabling the wilt virus. Forest stands of red oaks, with their tannin-rich foliage, may even provide caterpillars with safe havens from disease. In stands dominated by trees such as aspen, however, incipient gypsy moth outbreaks are quickly suppressed by viral epidemics.

Further research has shown that catefpillars become virtually immune to the wilt virus as the trees on which they 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 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' defensive tactic apparently has backfired.

Which of the following, if true, would most clearly demonstrate the operation of the trees` defensive mechanism as it is described in the first paragraph of the passage?

Caterpillars feeding on red oaks that were more than 50 percent defoliated grew to be only two-thirds the size of those feeding on trees with relatively intact foliage. Oak leaves in areas unaffected by gypsy moths were found to have higher levels of tannin on average than aspen leaves in areas infested with gypsy moths. The survival rate of gypsy moth caterpillars exposed to the wilt virus was 40 percent higher for those that fed on aspen leaves than for those that ate oak leaves. Female gypsy moths produced an average of 25 percent fewer eggs in areas where the wilt virus flourished than did moths in areas that were free of the virus. Gypsy moth egg clusters deposited on oak trees were found to have relatively large individual eggs compared to those deposited on aspen trees.

正确答案： A 耗时：