问题补充：
--- Can we leave the classroom now?
-- No, you . You to leave until the bell rings.
A. mustn’t, are allowed
B. don’t have to, are supposed
C. needn’t, aren’t allowed
D. can’t, aren’t supposed
答案：
A. mustn’t, are allowed
B. don’t have to, are supposed
C. needn’t, aren’t allowed
(答案→)D. can’t, aren’t supposed
解析：need，需要，否定式是needn’t；may可以，否定式是may not；can是能，能够，否定式是can’t。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:现在我可以离开教室了吗啊？——不，你不能。直到响铃你才能离开。短语be allowed to do sth被允许做某事。Be supposed to do sth应该做某事，被期待做某事，结合语境可知前一空表示客观上的不能，故用can’t，选D。
