Test takers remain in their seats until all the exam papers have been collected.A. canB.

问题补充：

Test takers remain in their seats until all the exam papers have been collected.

A. can

B. could

C. shall

D. would

答案：

A. can

B. could

(答案→)C. shall

D. would

解析：shall作情态动词用于第二、三人称时，表示“许可、允诺、命令、强制”，shall在该句中表“强制”。