GRE阅读 Passage 17
Early naturalists believed two species of beaver lived in North America: dam beavers and bank beavers. The bank species was thought to resemble the muskrat in behavior, living in burrows or lodges and unable to build dams. In fact, dams are primarily a strategy for dealing with annual variations in water levels. If water levels fall in summer, as they do in most of North America, then beavers lodge entrances may be exposed. With stabilized water levels, their homes are much safer. Along deep rivers, where bank beavers are found, this problem seldom arises. But these beavers do know how to build dams, and do so if the need arises, as may occur if they are forced to relocate after felling and consuming all nearby trees.文章解析
第一句话：
Early naturalists believed two species of beaver lived in North America: dam beavers and bank beavers. 白话版讲解： 早期的博物学家认为在北美洲有两种河狸：一种会叫“坝河狸”，一种叫“岸河狸”。 请注意这里使用了一般过去时。第二、第三、第四和第五句话：
The bank species was thought to resemble the muskrat in behavior, living in burrows or lodges and unable to build dams. In fact, dams are primarily a strategy for dealing with annual variations in water levels. If water levels fall in summer, as they do in most of North America, then beavers lodge entrances may be exposed. With stabilized water levels, their homes are much safer. 白话版讲解： 以前人们一般认为，“岸河狸”的行为模式像麝鼠，主要住在岸边的地下窝里，不会修坝（言下之意是以前人们一般认为“坝河狸”会修坝）。修坝是河狸为了应对河水水位每年（规律性的）涨跌而采取的一个策略。河狸的窝的洞口本来是位于水面以下的。每年夏天，北美大部分地区水位会下降，洞口就可能会露出来。这样就不安全了。如果河狸在自己窝附近的水里修个坝，就可以让自己窝附近的水域的水位稳定住（不下降），窝的洞口就不会露出来。第六和第七句话：
Along deep rivers, where bank beavers are found, this problem seldom arises. But these beavers do know how to build dams, and do so if the need arises, as may occur if they are forced to relocate after felling and consuming all nearby trees. 白话版讲解： “岸河狸”一般住在水比较深的河的河岸（旁边的水面下的洞里），水位很少降到低于它们的洞口。但是“岸河狸”并不是不会修坝（这句话否定了文章第二句话提到的“unable to build dams”这一观点），如果碰到确实需要修坝的情况，它们也是会修坝的。比如有时候，它们的窝附近的树被砍伐光和被烧光了，它们就不得不搬家，这时候它们就可能会修坝了。 注解： fell vt. 砍倒（树） cut down (a tree) 例句： A great number of trees were felled to provide space for grazing. 大量树木被砍伐，为放牧提供场地。 consume vt.(尤指火）毁灭，烧毁；荡尽 (especially of a fire) completely destroy 例句： The fire spread rapidly, consuming many homes. 火势蔓延得很快，烧毁了许多住宅。题目解析
1. The passage provides support for which of the following statements about beaver dams? A. One important function of these dams is to protect beavers homes. B. Most are built prior to burrow construction. C. They are found mostly along deep rivers. D. They are routinely abandoned as nearby forests are depleted. E. They mainly protect beavers from rising water levels.
白话版讲解：
问根据文章，关于河狸修的坝，下列哪一说法正确。 根据文章第三句话到第五句话，A选项正确。 文章没有比较建坝和建窝的时间先后，所以B选项错误。 根据文章，在水比较深的河，因为水位变化不会威胁到河狸洞口，所以坝是比较少的。所以C选项错误。 根据文章第八句话，树林被耗尽了，河狸将不得不搬家（放弃自己的窝），然后可能会修坝，不是放弃坝。所以D选项错误。 根据文章第四句话，水位下降才会危及河狸，所以应该是防止falling water levels。所以E选项错误。 Consider each of the choices separately and select all that apply. 2. The passage implies which of the following about beavers? A. Bank beavers are unable to successfully compete with dam beavers when resources become scarce. B. Differences in dam-building behavior among beavers do not necessarily imply multiple beaver species. C. Building dams eventually causes beavers to deplete nearby resources. D. When conditions permit, beavers are more likely to build dams than burrows or lodges. E. In beavers, dam-building is an acquired rather than an innate skill.
白话版讲解：
问根据文章，下列哪些说法正确？ 文章没有提到两种河狸的竞争，所以A错。 当你看到一群河狸在修坝，你是无法判断这是一群“岸河狸”还是“坝河狸”的，因为“岸河狸”遇到需要时也会修坝。所以B选项正确。 C选项把因果搞反了。 因为坝是用来保护洞和窝的，所以应该先修坝，所以D选项错误。 文章没有提到修坝是先天还是后天习得技能。所以E选项错误。 3. The passage provides support for which of the following statements about beaver dams? A. One important function of these dams is to protect beavers’ homes. B. They are less likely to be found along deep rivers. C. They are routinely abandoned before nearby forests are depleted.
白话版讲解：
问关于河狸修的坝，文章支持下列哪些说法？ A选项通过第1题A选项，正确。 B选项符合文章第七句话的“seldom”，正确。 C选项同第1题D选项，错误。 4. It can be inferred from the passage that early naturalists considered which of the following criteria when classifying beavers? A. The genetic similarity of beavers and muskrats B. The propensity of beavers to relocate C. The depth of water beavers occupied D. Beavers’ dam-building capabilities E. Beavers’ degree of exposure to danger
白话版讲解：
问早期博物学家本来以为用下来哪一个标准就可以把河狸分类？ 根据文章第一句话、第二句话，D选项正确。文章作者
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