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题目内容
（国家开放大学-开放英语4）
The world’s urban population, increasing at four times the rate of rural populations, will double in the period 1990 - 2025.
The world’s urban population, ____ ____ ____at four times the rate of rural populations, will double in the period 1990 - 2025.
The world’s urban population, ____ ____ ____at four times the rate of rural populations, will double in the period 1990 - 2025.
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