问题补充：
Students remain in their seats until all the papers have been collected.
A. may
B. shall
C. need
D. can
答案：
A. may
(答案→)B. shall
C. need
D. can
解析：考察情态动词用法。情态动词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。句意：学生们要坐在位置上直到所有的试卷都被收起来。根据句意说明这是按照考试的规定要做的事情。故B正确。
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