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A small number of the forest species of lepidoptera (moths and butterflies, which exist as caterpillars during most of their life cycle) exhibit regularly recurring patterns of population growth and decline—such fluctuations in population are known as population cycles. Although many different variables influence population levels, a regular pattern such as a population cycle seems to imply a dominant, driving force. Identification of that driving force, however, has proved surprisingly elusive despite considerable research. The common approach of studying causes of population cycles by measuring the mortality caused by different agents, such as predatory birds or parasites, has been unproductive in the case of lepidoptera. Moreover, population ecologists’ attempts to alter cycles by changing the caterpillars’ habitat and by reducing caterpillar populations have not succeeded. In short, the evidence implies that these insect populations, if not self-regulating, may at least be regulated by an agent more intimately connected with the insect than are predatory birds or parasites.
Recent work suggests that this agent may be a virus. For many years, viral disease had been reported in declining populations of caterpillars, but population ecologists had usually considered viral disease to have contributed to the decline once it was underway rather than to have initiated it. The recent work has been made possible by new techniques of molecular biology that allow viral DNA to be detected at low concentrations in the environment. Nuclear polyhedrosis viruses are hypothesized to be the driving force behind population cycles in lepidoptera in part because the viruses themselves follow an infectious cycle in which, if protected from direct sunlight, they may remain virulent for many years in the environment, embedded in durable crystals of polyhedrin protein. Once ingested by a caterpillar, the crystals dissolve, releasing the virus to infect the insect’s cells. Late in the course of the infection, millions of new virus particles are formed and enclosed in polyhedrin crystals. These crystals reenter the environment after the insect dies and decomposes, thus becoming available to infect other caterpillars.
One of the attractions of this hypothesis is its broad applicability. Remarkably, despite significant differences in habitat and behavior, many species of lepidoptera have population cycles of similar length, between eight and 11 years. Nuclear polyhedrosis viral infection is one factor these disparate species share.
【OG20-P431-536题】
According to the passage, before the discovery of new techniques for detecting viral DNA, population ecologists believed that viral diseases
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推断题,根据"population ecologists believed"定位到原文"but population ecologists had usually considered viral disease to have contributed to the decline once it was underway rather than to have initiated it."考的是长难句中rather than的取舍,意思是在已经decline的时候影响,而不是直接一开始就影响
A选项:原文提到"for many years, vital disease had been reported..."说明已经prevalent
B选项:对于only这种极端词一般都不选,仔细回归原文也没用提到lepidoptera的caterpillar life stage
C选项:信息混淆,原文中提到population cycle是population level的driving force,并没有提到viral diseases是population cycle的driving force,最后一句只提到了one factor
D选项:是对原文“have contributed to the decline once it was underway”的正确paraphrase
E选项:第一段已经提到predatory birds和parasites的猜想是unproductive的,而且viral diseases对birds和parasite的作用也没有提到
题目讨论 (2条评论)

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目标750
看到“new techniques for detecting viral DNA”,我们需要知道这句话在文中是起到什么作用的?与上下文什么关系。我们发现这句话是第二段的第二句,是用来支持前面的观点的,所以我们就需要看前面是什么观点,才能知道为什么讲这句话,才能知为什么引出这个“new techniques for detecting viral DNA”这个观点。 第二段第一句话的观点“For many years, viral disease had been reported in declining populations of caterpillars......”这句话看一半就够了意思很明确了,直接讲的是“这几年一直在报道病毒导致population下降”。 ;A:viral disease在 insect population中不泛滥(讲反了,都连年被报道了还不够泛滥吗?); ;B:只影响了毛虫的生命阶段(原文也没有这么讲,而且跑题,以后遇见这种“绝对性”的答案直接排掉就行,概率上99%都是错的,因为我就没见过对的); ;C:是导致population cycle的主要原因(是不是主要原因是出现在文章结尾的,不在这里,其次结尾也只是说明这是个 acceptible hypothesis); ;D:已经造成了大量的毛虫死亡(完全符合原文题意-原文“连着几年被报道 导致 declining population”); ;E:感染捕食毛虫的鸟和寄生虫(原文没讲virus感染鸟和寄生虫,而且和我们分析的原文内容不符,跑题严重)。
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0 回复 2021-06-11 23:20:30
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Joyce L
population ecologists had usually considered(before the discovery of new techniques for detecting viral DNA,) viral disease to have contributed to the decline once it was underway
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0 回复 2018-06-02 11:45:51