Rivers are one of the world's most important natural resources. Many cities are on large rivers, and almost every country has at least one river that plays an important part in the lives of its people. Besides transportation, rivers provide food, water for crops, water to drink, and opportunities for recreation for people who live along their (5)banks. And in order to get water for crops, engineers sometimes build a dam across a river and let the water become a lake behind the dam. Then people can use their water not only to irrigate fields but also to make electricity for homes and industries. (8)However, the water often becomes polluted when cities on river banks grow in size and the number of industries increases. We are learning that it is necessary to keep drivers clean if we want to enjoy the benefits of the natural resources. a) Are rivers important in people's lives? b) what do rivers provide people with? c) why is a dam built a cross a river? d) when are rivers polluted?