单项选择题

The Changjiang River is not the longest, the widest, or the most powerful river in the world. But in one sense it is the most important river, because it serves more people than any other river. In every way the Changjiang River is China’s life stream.
The Changjiang River isn’t just a trade river, a high way along which goods are picked up and sent. It is an agricultural river as well. A lot of irrigation(灌溉)ditches(渠)go out from it to millions of tiny garden-size farms. There men and women work endlessly with very old hand tools-planting, watering, fertilizing, weeding, harvesting, raising the family’s food and raising the nation’s food.
It begins somewhere high in the place north of Tibet, running down from a three-mile height. It goes for hundreds of miles, shouting loud through valleys. Only in the last 1,000 miles of its 3,900-mile journey does the Changjiang River become the sunny and cheerful river.
The upper part of the Changjiang River ______.

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