问题补充：
---Must we leave the classroom now?
---No, you . You leave after the bell rings.
A. mustn ; are allowed to
B. don have to; might
C. needn ; can
D. can ; are supposed to
答案：
A. mustn ; are allowed to
B. don have to; might
(答案→)C. needn ; can
D. can ; are supposed to
解析：must的含义是必须一定，其否定形式mustn’t表示禁止的含义。故must的否定回答一般借助于needn’和don’t have to来构成，句意:我们现在必须离开教室吗？——不，你们不必。你们可以在铃响之后离开教室。在表示可能时，might表示推测语气，can表示客观的能力，结合语境可知选C。
