7. 解析:选C。keep an eye on 照看，照顾;注意；have an eye for表示“对……有鉴赏力”。

6. 解析:选C。why引导定语从句修饰先行词the reason,表示原因。

5. 解析:选B。根据used to do可知，此处应选用表示过去的时间状语。at a time 一次；at all times 一直；at the same time 同时。故只有at one time(曾经)符合句意。

3. 解析:选C。go for在这里表示“向……进攻”。句意为：森林里的老虎从来不袭击人，除非它们太饿了。

2. 解析:选D。句意为：规章规定任何参与和外校学生打架斗殴的学生必须开除。be involved in干涉；参加。

5. The government has decided to get tough with people who avoided paying taxes.

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4. Nowadays, because of the destruction of the natural habitats, many rare animals are in danger of extinction.

3. When did that style of dress come into fashion?

2. She gave her life to helping the poor.