注意：当 need, want, require, worth(形容词)后面接 doing也可以表示被动。
Your hair wants cutting. 你的头发该理了。
The floor requires washing. 地板需要冲洗。
The book is worth reading. 这本书值得一读。
典型例题
The library needs___, but it'll have to wait until Sunday.
A. cleaning B. be cleaned C. clean D. being cleaned
答案 A. need(实意)+ n/ to do， need(情态)+ do，当为被动语态时，还可 need+ doing. 本题考最后一种用法，选 A。如有 to be clean则也为正确答案。
典： done，"不可能已经"。 must not do不可以(用于一般现在时)。
