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托福阅读句子插入题：Symbiotic Relationships

2015-08-21 16:47:55来源：网络

　　【托福阅读题型：Symbiotic Relationships句子插入题】托福阅读中句子插入题是重要题型之一。根据托福阅读题型不同来进行专项练习，是短期提高托福阅读能力的有效方法。同学们需要多加练习哦!

　　查看全部：托福阅读题型分类练习之句子插入题

　　托福阅读句子插入题：Symbiotic Relationships

　　At times, it is actually possible to watch the effects of natural selection in host-parasite relationships. For example,　Australia during the 1940 s was overrun by hundreds of millions of European rabbits. ■ The rabbits destroyed huge　expanses of Australia and threatened the sheep and cattle industries. ■ In 1950, myxoma virus, a parasite that affects　rabbits, was deliberately introduced into Australia to control the rabbit population. ■ Spread rapidly by mosquitoes, the　virus devastated the rabbit population. ■ The virus was less deadly to the offspring of surviving rabbits, however, and it　caused less and less harm over the years. Apparently, genotypes (the genetic make-up of an organism) in the rabbit　population were selected that were better able to resist the parasite. Meanwhile, the deadliest strains of the virus　perished with their hosts as natural selection favored strains that could infect hosts but not kill them. Thus, natural　selection stabilized this host-parasite relationship.(Symbiotic Relationships)

　　! Look at the four squares [■] that indicate where the following sentence could be added to the passage.

　　This massive population began a century earlier as a mere twelve pairs of imported rabbits that reproduced quickly

　　and developed into a major problem.

　　Where would the sentence best fit?

　　答案见下一页：

本文关键字: 托福阅读题型 托福阅读训练 句子插入题

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