In our class, when the bell rang and the teacher closed his book, it was a for everyone to stand up.
A. signal
B. chance
C. mark
D. measure
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(答案→)A. signal
B. chance
C. mark
D. measure
解析：根据句意可知打铃及老师合上书本，这是下课的暗示，选项中只能选择signal。chance“机会”，mark“分数，痕迹”，measure“方法，措施”。