all the homework assigned by the school the students have to work at it till midnightA. C

问题补充：

all the homework assigned by the school the students have to work at it till midnight

A. Completing

B. To complete

C. Having completed

D. To have completed

答案：

A. Completing

(答案→)B. To complete

C. Having completed

D. To have completed

解析：句意：为了完成学校布置的全部的家庭作业，学生们不得不学习到了午夜。表示目的，为了……，故用不定式，且与谓语动词表示的动作没有时间的先后顺序，故用不定式的一般式，选B。