It has been announced that all the candidates___remain in their seats until all the papers have been collected.

问题描述:

It has been announced that all the candidates___remain in their seats until all the papers have been collected.
A. shallB. should
为啥选A? 求区分这两个.→_→

A
shall 和类似 all the candidates这样的三人称连用,带有强制意味,没有商榷余地,如条例,规章等的要求
should 表示主观上认为应该怎么做