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)。