电气设备的外壳应有什么防护措施？（）
A．无
B．保护性接地
C．防锈漆
D．绝缘
参考答案：B
磨除塑料废边常规使用的磨具是
A．裂钻球钻 B．石英粉 C．白毛刷 D．布轮 E．大砂轮
It was not until she got home ______ Jennifer realized she had lost her keys.
A．when
B．that
C．where
D．before
