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

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：B 考察非谓语动词。句义：为了完成学校布置的所有作业，学生们不得不写作业到深夜。根据句义可知此处用动词不定式做目的状语。故B正确。