Rivers all of the world’s most important natural resources. Many cities are on large rivers, and(24)__________ every country has at least one river that plays an important part in the lives if its people. Besides transportation, rivers(25) _________ food , water for crops , water to drink and opportunities for recreation for people who live along their bank and in order to get water for crops engineer sometimes build a dam(26)__________ 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 home and industries. However the water often becomes(27)__________ when cities on river bank grow inside and the number of industries increases. We are learning that it is necessary to keep rivers clean if we want to enjoy the(28) ________ of the natural resources.