Rivers are on of the world's most important natural resources. Many cities are on the large rivers, and (1) ______ every country has a least one river that (2)______ an important part in the lives of its people. Besides transportation, rivers (3)______ water for crops, water to drink, and recreation for people who live along their (4)______ And in (5)______ to get water for crops, engineers sometimes build a dam (6)______ a river and let water become a lake behind the dam. Then people can use their water not (7)______ to irrigate fields but also to make electricity for homes and industries. (8)______ the water often becomes polluted when cities on river bank grow (9)______ and the number of industries increases. We are learning that it is necessary to (10______ rivers clean if we want to enjoy the benefits of the natural resources.
