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阅读理解
请认真阅读下列短文，从短文后备题所给的A、B、C、D四个选项中，选出最佳选项。
You are about to say something to your younger brother when he gestures that he’s got a phone message．
“But I didn’t hear the ring, ”you say．
“Because you are too old, ”your brother answers．
Don’t get mad．He’s just telling you the truth．
Teenagers in Britain and the US have a weapon against the older generation, especially parents and teachers．It’s a ring tone that many adults cannot hear．
According to a recent report from the New York Times, in some Manhattan schools, students have begun using the technology．Cellphone use is usually forbidden in class．But using the high-pitched(频率)ring tone, students can successfully exchange phone messages without being found by their teachers．
The technology relies on the fact that most adults gradually lose the ability to hear high-pitched sounds, It was developed in Britain but has recently spread to America by the Internet．
However, the technology was originally aimed at annoying teenagers, not the other way around．
Called the Mosquito, it was marketed last year by a Welsh security company．It is an ear-splitting 17-kilohertz buzzer(蜂鸣器)designed to help shopkeepers get rid of young people hanging around in front of their stores．Humans can hear sounds taking place in a frequency range between 200 and 20,000 hertz, as the research shows．But our hearing gradually becomes worse when we grow up．
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