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
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。