【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正确答案D
