Water has been vital to the development and survival of civilization. The first great civilizations arose in the (1)______ of great rivers. They built large irrigation systems, made the land productive, and prospered. The challenge of today, as in ancient times, is for people to make the best use of water. But the challenge is greater than ever before because more water is needed as the world's (2)______ increases. Scientists estimate that nearly 50 countries will face water shortages by 2025. Also, many people do not conserve water, and they (3)______ water and manage it poorly in other ways. Countries are working together to try (4)______ water problems. The United Nations has been heavily involved in these efforts. In addition, groups of countries (5)______ lands are drained by major rivers and seas have formed regional organizations to fight water pollution. 1. A. valleys B. water C. bottoms D. front 2. A. people B. inhabitants C. residents D. population 3. A. use up B. drain C. pollute D. irrigate 4. A. solving B. to solve C. explaining D. to explain 5. A. their B. what C. whose D. which