单选题

　　[概括大意]Radar is now a familiar tool. Like many others it was an unexpected discovery. It was first observed by two researchers， who were studying sound communication. They were sending signals from a station on one side of a river in Washington D to a vehicle across the river. They discovered that their signals were stopped by passing ships. They recognized the importance of this discovery at once.

　　The main idea of this paragraph is__________.

　　A.Study of Radar

　　B.Radar is now a familiar tool

　　C.Radar Working Principles

　　D.Radar was Discovered by Chance