【概括大意】Radar is now a familiar tool. Like many others it was an u

【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__________.

Study of Radar

Radar is now a familiar tool

Radar Working Principles

Radar was Discovered by Chance

unexpected，“没有预见到的”，等于答案D中的by chance“偶然间的”。这道题用到了同义词替换这一解题方法。