_____the exercise-books_____(must return)to the students tomorrow?

题目

_____the exercise-books_____(must return)to the students tomorrow?
请分析
完整正确。be returned，为什么用这个？

答案

must the exercise books be returned to the students tomorrow?这些练习册明天必须要发给学生们么?be returned,表示有返还的意思,也许这个句子有前因表示这些练习册是学生交上来的作业,现在由老师批阅过发还给学生...