轻瓦斯动作后收集到的气体为无色无味不易燃的气体时，则可判断变压器内部（）。
A.相间短路；
B.铁心片间短路；
C.单相接地；
D.无故障。
参考答案：D
管道设备的换气工作可以在夜间进行。
20．It was four o'clock in the afternoon_____he and his grandpa reached the museum in Guanghan, _____an official warmly received them.
A．that; where
B．when; that
C．when; where
D．that; when
