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：signal信号；chance机会；mark标志；measure测量。句意是在我们课堂上，当铃声响起，老师合起课本时，这是让每个人都起立的信号。故选A。