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。