问题补充：
It has been announced that students remain in their seats until all the papers have been collected.
A. can
B. may
C. will
D. shall
答案：
A. can
B. may
C. will
(答案→)D. shall
解析：本题考查情态动词在这句话中，正确的句意应该为：学生必须留在座位上，直到所有的试卷都被收完为止。shall用作情态动词时，主语可以为第二或者第三人称，表示说话者对主语的命令、要求或者承诺等等，最符合本句的意思。而may/ can表允许；will表意愿，均不符合题意，故排除。
