it has been decided that all the students______stay in the classroom ever on saturday evenings in case they go to intern
问题描述：
it has been decided that all the students______stay in the classroom ever on saturday evenings in case they go to internet bars
A.MUST B.CAN C.NEED D.SHALL
选哪个?为什么?
我迷糊了
答
A 必须 是规定