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完形填空

　　Rivers can be young, 1 and old. Young rivers are the 2 that are busy 3 valleys shaped like the letter V 4 they rush down from mountain. The 5 water and pieces of rock 6 down the river and rub away the land. These pieces of rock become 7 rounded stones and sand, soil is washed 8 the river in the 9 of mud.

　　After many 10 years, the mountains and the slopes(斜坡) on 11 side of the river 12 worn down (磨掉). As valley becomes 14 and its shape looks more 15 the letter U. This is a middle-aged river.

　　An 16 river has almost finished the job of wearing away the land. The land along 17 banks is 18 . The water moves 19 , except at flood times, and 20 only fine mud. It winds(蜿蜒)like the letter S through the wide plain(平原).

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