接地电网接地电阻值测定周期为（）一次。
A.每月
B.每季
C.每半年
参考答案：B
若限速器装在井道内，则应能从井道外面接近它。（）
You _____ buy a gift, but you can if you want to. [ ]
A. must
B. mustn't
C. have to
D. don't have to
