题目内容
() 4. We mustn't go into the teachers’ office without the door.
A. knock on
B. knocked on
C. to knock
D. knocking on
4.D
4.D
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题目内容
() 4. We mustn't go into the teachers’ office without the door.
A. knock on
B. knocked on
C. to knock
D. knocking on
4.D