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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 (免疫学家)Mark Laudenslger, 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 mot. 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.
注：1. vulnerable易受伤害的

2. immune defense 免疫系统

3. found之后是试验告诉我们的观点

4. depress压抑，de往下

 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.

注：1. 看第一句三个单词就知道本段内容和上一段相同。

2. passive被动的

21. Laudenslager's experiment showed that the immune system of those rats who could turn off the electricity ______.
 A) was strengthened

 B) was not affected
 C) was altered

 D) was weakened

注：定位文章第一段found之后

22. According to the passage, the experience of helplessness causes rats to ______.
 A) try to control unpleasant stimuli
 B) turn off the electricity
 C) become abnormally suspicious

 D) behave passively in controllable situations
注：定位第二段But之后那句话

23. The reason why the mice in Ader's experiment avoided saccharin was that ______ .
 A) they disliked its taste
 B) it affected their immune systems
 C) it led to stomach pains
 D) they associated it with stomachaches

22. The first paragraph is mainly about _____.

 (A) the teenagers' criticism of their parents

 (B) misunderstandings between teenagers and their parents

 (C) the dominance of the parents over their children

 (D) the teenagers' ability to deal with crises