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.

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6-12岁英语高频重点词汇，搞定小学英语全段考试

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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。

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