Điền từ thích hợp vào chỗ trống

Rivers are one of the world's most important natural resources. Many cities are on large rivers, and (23) _______every country has at least one river that plays an important part in the lives of its people.

          Besides transportation, rivers (24) _______ food, water for crops, water to drink, and opportunities for recreation for people who live along their banks. And in order to get water for crops, engineers sometimes build a dam (25) _______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.

          However, the water often becomes (26) _______ when cities on river banks grow in size and the number of industries increases. We are learning that it is necessary to keep rivers clean if we want to enjoy the (27) _______of the natural resources.