题目内容：

Rivers are one of our most important natural resources. Many of the world’s great cities are 1.______ (locate) on rivers, and almost 2._______ country has at least one river flowing 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 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 are learning the 9._________ (important) of doing more to keep their rivers clean 10.________ they want to enjoy the benefits of this natural resource.