It has been announced that all the candidates remain in their seats until all the papers h

问题补充：

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

A. can

B. will

C. shall

D. should

答案：

A. can

B. will

(答案→)C. shall

D. should

解析：考查情态动词。 shall 用在二、三人称当中表示命令、允诺、规定等，在这里表示一种规定。