问题补充：
---- May we leave the classroom now?
---- No, you . You to leave until the bell rings.
A. mustn’t, are allowed
B. can’t, aren’t supposed
C. don’t have to, are supposed
D. needn’t, aren’t allowed
答案：
A. mustn’t, are allowed
(答案→)B. can’t, aren’t supposed
C. don’t have to, are supposed
D. needn’t, aren’t allowed
解析：考查情景交际和情态动词。“我们现在可以离开教室了吗？”“不，你们不可以。你们不能离开，在铃声响起之前。”can’t表示允许，“不可以”；needn’t“不必”；be supposed to do sth.“应该做某事”。故选B。
