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(免疫學(xué)家) 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 immune response was discovered by chance. In 1975 psychologist Robert Ader at the University of Rochester School of Medicine conditioned(使形成條件反射) mice to avoid saccharin(糖精) by simultaneously feeding them the sweetener and injecting them with a drug 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 saccharin, 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.

　　21. Laudenslager's experiment showed that the immune system of those rats who could turn off the electricity .

　　A) was altered C) was weakened

　　B) was strengthened D) was not affected

　　22. According to the passage, the experience of helplessness causes rats to .

　　A) turn off the electricity

　　B) try to control unpleasant stimuli

　　C) become abnormally suspicious

　　D) behave passively in controllable situations

　　23. The reason why the mice in Ader's experiment avoided saccharin was that.

　　A) they associated it with stomachaches

　　B) it affected their immune systems

　　C) it led to stomach pains

　　D) they disliked its taste

　　24. The passage tells us that the most probable reason for the death of the mice in Ader's experiment was that .

　　A) the sweetener was poisonous to them

　　B) their immune systems had been altered by the mind

　　C) they had been weakened psychologically by the saccharin

　　D) they had taken too much sweetener during earlier conditioning

　　25. It can be concluded from the passage that the immune systems of animals .

　　A) can be altered by electric shocks

　　B) can be weakened by conditioning

　　C) can be suppressed by drug injections

　　D) can be affected by frequent doses of saccharin

　　21.D 22.D 23.A 24.B 25.B