______ arrival at the small village, they caught sight of the thick green trees in and around it and were attracted by the beautiful scenery .

A．On

B．At

C．In

D．As

电流继电器与电流互感器若采用三相三继电器安全星形接线，当所保护线路发生任何形式的短路故障时，都能起到（）。