It has been announced that candidates remain in their seats until all the papers have been collected.
A. shall
B. can
C. should
D. must
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(答案→)A. shall
B. can
C. should
D. must
解析：考察情态动词用法。本题考察了情态动词shall的特殊用法，shall还可以可用于二、三人称的肯定句中表示说话人给对方的命令、警告、允诺、 威胁等。也可以用于根据法律法规必须做某事。如According to law, you shall give all your money to your daughter。本题正是考试的规则。句意：据宣布，所有的考生要坐在原来的位置上直到所有的试卷都被收起来。故A正确。