仪表用电缆、电线、补偿导线的敷设与保温的工艺设备、工艺保温层表面之间的距离要大于（）mm。
A.100
B.200
C.250
D.400
参考答案：B
Not until he left his home ______ to know how important the family was for him.
A．did he begin
B．had he begun
C．he began
D．he had begun
三相异步电动机产生过载的原因主要有（）。
A、启动时间太长
B、人为的原因
C、电网电压太低
D、机械故障
