Yet wireless communications in these areas are advancing rapidly at a time when the mobile-phone industry is approaching saturation point in rich countries and the average revenue per user is sagging.

然而，当移动电话产业在发达国家一度接近饱和，而且从每位用户取得的收入开始下跌的时候，无线通讯在这些地区依然飞速发展。

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The frequency of a sound wave, perceived as pitch, is the number of compressions that pass a fixed point per unit time.

声波的频率就是感觉的音调，是单位时间通过定点的压缩数目。

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Income is a flow of earnings per unit of time. Money, by contrast, is a stock: it is a certain amount at a given point in time.

收入是某一单位时间内收益的流量，而货币是一个存量，即某一时间点上一个特定的金额。

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