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.