Studies of the Weddell seal in the laboratory have described the physiologic

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问题 Studies of the Weddell seal in the laboratory have described the physiological mechanisms that allow the seal to cope with the extreme oxygen depriva- tion that occurs during its longest dives, which can extend 500 meters below the ocean’ s surface and last for over 70 minutes. Recent field studies, however, suggest that during more typical dives in the wild, this seal’ s physiological behavior is different.
In the laboratory, when the seal dives below the surface of the water and stops breathing, its heart beats more slowly, requiring less oxygen, and its arteries become constricted, ensuring that the seal’ s blood remains concentrated near those organs most crucial to its ability to navigate underwater. The seal essentially shuts off the flow of blood to other organs, which either stop functioning until the seal surfaces or switch to an anaerobic(oxygen-independent)metabolism. The latter results in the production of large amounts of lactic acid which can adversely affect the PH of the seal’ s blood but since the anaerobic metabolism occurs only in those tissues which have been isolated from the seal’s blood supply, the lactic acid is released into the seal’ s blood only after the seal surfaces, when the lungs, liver, and other organs quickly clear the acid from the seal’s blood stream. Recent field studies, however, reveal that on dives in the wild, the seal usually heads directly for its prey and returns to the surface in less than twenty minutes. The absence of high levels of lactic acid in the seal’ s blood after such dives suggests that during them, the seal’ s organs do not resort to the anaerobic metabolism observed in the laboratory, but are supplied with oxy- gen from the blood. The seal’ s longer excursions underwater, during which it appears to be either exploring distant routes or evading a predator, do evoke the diving response seen in the laborato- ry. But why do the seal’ s laboratory dives always evoke this response, regardless of their length or depth? Some biologists speculate that because in laboratory dives the seal is forcibly sub- merged, it does not know how long it will remain underwater and so prepares for the worst. [br] Which of the following best summarizes the main point of the passage?

选项 A、Recent field studies have indicated that descriptions of the physiological behavior of the Weddell seal during laboratory dives are not applicable to its most typical dives in the wild.
B、The Weddell seal has developed a number of unique mechanisms that enable it to remain submerged at depths of up to 500 meters for up to 70 minutes.
C、The results of recent field studies have made it necessary for biologists to revise previous perceptions of how the Weddell seal behaves physiologically during its longest dives in the wild.
D、Biologists speculate that laboratory studies of the physiological behavior of seals during dives lasting more than twenty minutes would be more accurate if the seals were not forcibly submerged.
E、How the Weddell seal responds to oxygen deprivation during its longest dives appears to depend on whether the seal is searching for prey or avoiding predators during such dives.

答案 A

解析主题类型：A．正确。最近野外研究指出：对于威德尔海豹在实验室中潜水时生理行为的描绘不适用于野外更典型的潜水。第一段的总结和其后全文的意思都是如此。B．威德尔海豹的独特机制使其能潜水500米深，70分钟长。显然，这是实验室研究内容，作者对此提出了不同意见。C．“during its longest dives”所说和原文不符，作者新研究成果是“most typical dives”。D．当海豹不被迫潜水，实验室结果会有效些。说反了。恰恰是被迫潜水时才和实验室中结果一样。E．此选项意思不通：最长时间潜水时海豹怎样对待缺氧问题取决于捕食还是逃跑。

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