核心问题
（1）Sir, you ____ sit here. It's for women and children only.
A. don't B. can't C. could D. needn't(南京中考)
(2) Sir, you ____ be sitting in this waiting room. It is for women and children only. A. oughtn't to B. can't C. won't D. needn't(上海高考,96-23)
试题(1)的答案是 B, 而试题(2)的答案却是 A。我认为这两个答案是互相矛盾的。
词典解答
试题(1)选B是正确的。在试题(2)中，在没有上下文的条件下，选A或选B都是正确的[见 ]，只是语气有所不同。选A，语气显得客气、委婉，选B，语气则有些生硬。但是，上面的句子没有语境限定，使用A或B都应看作是正确的（注意：用了Sir，不一定非使用客气的语气不可）。因此，我们有理由说，上面的试题大有改进的必要。
cannot have done 见 can
cannot help it 见 cannot/ couldn't help it
cannot...too 见 can