【OG24-743题】When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following aperiod of extreme drought, algae levels are low even in very slow-moving river water.Which of the following, if true, does most to explain the contrast described above? _mofa留学圈mofaGRE在线题库系统_GRE在线搜题_GRE题库解析_GRE真题机经_GRE课程_GRE培训机构

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When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following aperiod of extreme drought, algae levels are low even in very slow-moving river water.

Which of the following, if true, does most to explain the contrast described above?

During periods of extreme drought, the populations of some of the species that feed on algae tend to fall. The more slowly water moves, the more conducive its temperature is to the growth of algae. When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there. Australian rivers dry up completely for short intervals in periods of extreme drought. Except during periods of extreme drought, algae, levels tend to.be higher; in rivers in which the flow has been controlled by damming than in rivers that flow freely.

正确答案： D

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题干翻译
因为栖息地是河水的藻类在水流缓慢时生长最好，所以通常在降雨很少时，单位水量中的藻类数量会增加。然而，相比之下，在一段极端干旱时期之后，即使在水流非常缓慢的河水中，藻类水平也很低。 逻辑链
little rain→algae↑；extreme drought→algae↓ 问题类型
解释题 推理过程
需要找到一个原因来解释为什么在降雨少（水流缓慢利于藻类生长）时藻类数量增加，而在极端干旱后即使水流缓慢藻类数量却很低。 选项分析
A选项：在极端干旱时期，一些以藻类为食的物种数量往往会下降。这应该会使藻类数量增多，而不是减少，无法解释矛盾，排除。 B选项：水流移动越慢，其温度对藻类生长越有利。这只能说明水流慢与藻类生长的关系，不能解释极端干旱后藻类少的情况，排除。 C选项：当藻类种群达到非常高的水平时，河流内的条件对一些通常生活在那里的其他物种可能变得有毒。讨论的是藻类对其他物种的影响，与极端干旱后藻类数量低无关，排除。 D选项：在极端干旱时期，澳大利亚的河流会在短时间内完全干涸。干涸导致藻类无法生存，所以即使之后水流缓慢，藻类数量也低，能够解释矛盾，当选。 E选项：除了在极端干旱时期，在水流被大坝控制的河流中的藻类水平往往比自由流动的河流中的藻类水平更高。比较的是不同河流类型在非极端干旱时的藻类水平，无法解释极端干旱后的情况，排除。

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