Question

In the street，you should be careful enough ________ old people down.

A. avoid knocking    B. avoiding to knock

C. to avoid knocking    D. to avoid to knock

Answer

C

【解析】句意：在大街上，你应该足够小心避免撞倒老人。根据句意，结合固定搭配：be careful enough to do sth,足够小心做某事；avoid doing，避免做某事。故答案为：C.

点睛： enough用作副词，可用来修饰形容词、副词、动词等，此时总是放在被修饰语之后，同时后面使用to do不定式表示结果。