of economic development along the river. Research on some dead baiji collected in recent years showed that more than 90

　　percent of the deaths were caused by human activities.

　　One reason is that over­fishing has cut the dolphin's food sources. Dams built along the river and neighboring waterways have also kept them from swimming into and out of their habitats. Meanwhile, waste discharge (排放) has seriously

　　polluted the river. And heavy river traffic has greatly reduced their living space.

　　The disappearance of the baiji would be the first instance of a large aquatic mammal (水生哺乳动物) being driven to

　　extinction since hunting killed off the monk seal (僧海豹) around 1952.

　　本文是篇说明文，主要对白鳍豚这一物种进行介绍，并指出过度捕鱼、修建大坝和废物排放是导致这一物种消亡的主要原因。

　　51．Baiji dolphin used to live on the

　　________

　　of the Yangtze River.

　　A．upper sections

　　B．middle sections

　　C．lower sections

　　D．middle and lower sections

　　答案：D　细节理解题。根据第二段第一句中的“any sign of the freshwater dolphin on the middle and lower sections of the river”可知，白鳍豚生活在长江中下游地区。故选D项。

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