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听力习题：

　　33.

　　A. If they don't involve any risks.

　　B. If they produce predictable side effects.

　　C. When the urgent need for them arises.

　　D. When tests show that they are relatively safe.

　　34.

　　A. Because they are not accustomed to it.

　　B. Because they are not psychologically prepared for it.

　　C. Because their genes differ from those who have been tested for it.

　　D. Because they are less sensitive to it than those who have been tested for it.

　　35.

　　A. They will have to take ever larger doses.

　　B. They will become physicaly impaired.

　　C. They will suffer from minor discomfort.

　　D. They will experience a very painful process.

【责任编辑：屈应兰】