概括大意与完成句子
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 DC. 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
【答案解析】 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。
