Ten years ago, I had worked as a night guard in a factory. On a rainy night, all the lights of factory went out all of a sudden. I rushed out and could see nothing. Lucky for me, the warning system was directly connected with the police station. Three or four minutes late, four police car arrived at the front gate. They searched for the factory carefully, tried to find something unusual. In the end, I am told that the rain had brought about   the short circuit, that caused the accident.

城域网的主干网采用的传输介质主要是______。

A．同轴电缆

B．光纤

C．屏蔽双胶线

D．无线信道