12.7 need/want/require/worth
  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。

  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)。