5. In our class when the bell rang and the taacher closed his book it was a or everyone

问题补充：

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

答案：

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”。