电空阀是一种借助于电磁吸力的作用来控制压缩空气管路的（），以实现气路转换的三通阀门。
A、阻断
B、接通
C、接通或阻断
D、转换
参考答案：C
The doctor _____ carefully and found nothing serious. [ ]
A. looked for him
B. looked him over
C. looked at him
D. looked him after
---When did you arrive the day before yesterday?
--_________2:30p.m.________3:00p.m.
A．From; to
B．At; at
C．Between ;and
D．/;/
