Students remain in their seats until all the exam papers have been collected.A. mustB. sh

问题补充：

Students remain in their seats until all the exam papers have been collected.

A. must

B. shall

C. may

D. can

答案：

A. must

(答案→)B. shall

C. may

D. can

解析：考查情态动词shall的用法。Shall用于二三人称时表示一种语气，如命令、允诺、规定、警告等等。句意：学生们要保持坐在自己的座位上直到所有的试卷被收齐。Shall在这里表示一种规定。所以选B。