问题补充:
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
答案:
(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”。