题目内容：

Rivers are one of our most important natural resources. Many of the world’s great cities are located on rivers, and almost _1.______ country has at least one river _2._____(flow) through it that plays 3._______ important part in the lives of its people.

Since the beginning of history, people 4.______________ (use) rivers for transportation. The longest one in the United States is the Mississippi, and the lifeline of Egypt is the Nile. __5.___________ transportation, rivers give water to drink, water for crops, and chances for fun and recreation （消遣，娱乐活动）for the people __6.______________ live along their banks.

However, large cities and industries that are located on rivers often make problems. As the cities grow _7.___________ size and industries increase in number, the water in the rivers becomes 8.__________ (pollute) with chemicals and other materials. People come to kwon the _9.____________ (important) of doing more to keep their rivers clean _10.____________ they want to enjoy the benefits of this natural resource.