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     Do you live in a city? Do you know how cities began? Long long ago, the world had only a few thousand people. These people moved from one place to another. They moved over the land, hunting animals for food. No one knows how or when these people learned about growing food. But when they did, their lives changed. They did not have to move houses any more. They could stay in one place and grow it. People began to live near each other, so the first village grew. Many people came to work in the villages, and these villages became larger and larger. Then people had machines, life in the villages changed again. People built factories. More and more people lived near the factories. So the villages grew into cities. Some of them grew very big. Today, it’s strange that some people are moving back to small villages. Can you tell me why ?

分布在干旱地区，防风固沙  ____   A湿地生态系统

净化水质，蓄洪抗旱        ____   B草原生态系统

有“绿色水库”之称        ____   C海洋生态系统

制氧量最多                ____   D森林生态系统

消费者主要是人            ____   E城市生态系统

将少量的①粉笔灰.②煤油.③食盐晶体.④硫酸铜晶体.⑤高锰酸钾晶体分别放入足                 量的水中，充分搅拌，不能形成溶液的是(填编号，下同)____，能形成无色溶液的是  ____，能形成蓝色溶液的是____，能形成紫红色溶液的是____ .