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  3. We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower animals to

We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower animals to

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问题    We sometimes think humans are uniquely vulnerable to anxiety, but stress seems to affect the immune defenses of lower animals too. In one experiment, for example, behavioral immunologist Mark Laudenslager, at the University of Denver, gave mild electric shocks to 24 rats. Half the animals could switch off the current by turning a wheel in their enclosure, while the other half could not. The rats in the two groups were paired so that each time one rat turned the wheel it protected both itself and its helpless partner from the shock. Laudenslager found that the immune response was depressed below normal in the helpless rats but not in those that could turn off the electricity. What he has demonstrated, he believes, is that lack of control over an event, not the experience itself, is what weakens the immune system.
   Other researchers agree. Jay Weiss, a psychologist at Duke University School of Medicine, has shown that animals who are allowed to control unpleasant stimuli don’t develop sleep disturbances or changes in brain chemistry typical of stressed rats. But if the animals are confronted with situations they have no control over, they later behave passively when faced with experiences they can control. Such findings reinforce psychologists’ suspicions that the experience or perception of helplessness is one of the most harmful factors in depression.
   One of the most startling examples of how the mind can alter the immuue response was discovered by chance. In 1975 psychologist Robert Ader at the University of Rochester School of Medicine conditioned mice to avoid saccharin by simultaneously feeding them the sweetener and injecting them with a drag that while suppressing their immune systems caused stomach upsets.  Associating the saccharin with  the stomach pains, the mice quickly learned to avoid the sweetener. In order to extinguish this dislike for the sweetener, Ader reexposed the animals to sac charin, this time without the drug, and was astonished to find that those mice that had received the highest amounts of sweetener during their earlier conditioning died. He could only speculate that he had so successfully conditioned the rats that saccharin alone now served to weaken their immune systems enough to kill them.
The passage tells us that the most probable reason for the death of the mice in Ader’s experiment was that ______.
选项 A、they had been weakened psychologically by the saccharin
B、the sweetener was poisonous to them
C、their immune systems had been altered by the mind
D、they had taken too much sweetener during earlier conditioning
答案C
解析 作者写第三段的目的是为了证明他在第三段第一句里的论点"the mind can alter the immune response"，他用Ader的实验这一实例米论证此论点。为了培养老鼠的条件反射，Ader给它们喂食糖精，同时注射能降低免疫力并可引起肚子疼的药物，但后来只给老鼠喂食糖精而不注射药物时，有些老鼠死亡，这允分证明老鼠的免疫系统已被其思想改变了，所以[C]是正确答案。
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