单项选择题

Questions 18 to 20 are based on the following, passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage. The practical use of radio waves brought about______.

A．new machines for planes
B．a radio directing and ranging device
C．new radio sets for RAF bombers
D．a discovering device

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单项选择题

单项选择题

Questions 18 to 20 are based on the following, passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage. The practical use of radio waves brought about______. 您的浏览器不支持 audio 元素。