问题补充：
—May we leave 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：...until...是“直到”的意思,表示某一种行为一直持续到某一时间；...not...until是“直到...才”表示直到某一时间,某一行为才发生,而在之前该行为并没有发生.如: I studied English until 9 o’clock last night. 我咋晚学英语一直学到9点钟(表示9点前一直在学) 。I did not study Englis until 9 o’clock last night. 我咋晚直到9点钟才学英语(表示9点才开始学)；needn’t:不必；can’t:不能。根据句意，应是“你不能离开，直到铃响才能离开”故选D。
