5. In our class, when the bell rang and the taacher closed his book, it was a or everyone to stand up.
A. signal
B. chance
C. mark
D. measure
网友回答
A. signal
B. chance
C. mark
D. measure
解析：
signal 信号； chance 机会； mark 记号；measure 措施。句中的it 代替“铃响时教师合上书”这件事，而“它”正是大家要起立的“信号”。