污闪电压
参考答案：
表面脏污的绝缘子在受潮的情况下的闪络电压。
计算 之于（ ）相当于 实验 之于（ ）
A．推理;逻辑
B．物理;天文
C．数学;化学
D．大脑;助手
By the time Jack returned home from England, his son________from college.[ ]
A. graduated
B. has graduated
C. had been graduating
D. had graduated
