为防止介质沿阀杆漏出，可在截止阀阀杆穿出阀盖部位用（）来密封。
A、机封
B、填料
C、垫片
D、密封胶
参考答案：B
—The box is too heavy! I can't carry it.
― ! Let me help you. [ ]
A. Don't worry
B. That's OK
C. Don't be afraid
D. Thanks a lot
电力系统在运行中，突然短路引起的过电压叫做（）过电压。
A．大气
B．操作
C．弧光接地
