为防止电网与地短路和漏电危险，保护人员免受触电危险，必需采用（）保护装置。
A、短路
B、断电
C、过流
D、过压
参考答案：B
His sister had to wait for him for another two hours, _______ she? [ ]
A. wasn't
B. doesn't
C. hadn't
D. didn't
CA6140型车床溜板箱内的安全离合器可在车螺纹时起保护作用。( )
