带电作业绝缘工具的电气试验周期是（）。
A、一年一次；
B、半年一次；
C、一年半一次；
D、两年一次。
参考答案：B
—Mary was being badly treated when John happened to pass by and helped her out.
—I think it was not a pure ________but had been well planned.
A．coincidence
B．consequence
C．incident
D．contradiction
为什么轴承进油管细而回油管粗？
