Rivers are one of our most important natural resources. Many of the world’s great cities are located on rivers, and almost _______ country has at least one river ______(flow) through it that plays _______ important part in the lives of its people.
Since the beginning of history, people ______________ (use) rivers for transportation. The longest one in the United States is the Mississippi, and the lifeline of Egypt is the Nile. _____________ transportation, rivers give water to drink, water for crops, and chances for fun and recreation （消遣，娱乐活动）for the people ________________ live along their banks.
However, large cities and industries that are located on rivers often make problems. As the cities grow ____________ size and industries increase in number, the water in the rivers becomes __________ (pollute) with chemicals and other materials. People come to kwon the _____________ (important) of doing more to keep their rivers clean _____________ they want to enjoy the benefits of this natural resource.