农村低压电网选用二级保护时，为确保保护器动作的选择性，（）必须选用延时型剩余电流动作保护器。
A.末级保护
B.总保护
C.总保护和末级保护
参考答案：B
_____ several times, he still could not make sense of it. [ ]
A. Having been told
B. Told
C. I le was told
D. Being told
电影票“9排21号”若记作（9，21），则（21，7）表示______．
