听下面一段材料，回答1-4题。 1. How many American rivers are seriously polluted? A. One-sixth of them. B. One-third of them. C. Half of them. 2. When did Susan start the work of cleaning the river? A. Three years ago. B. Last August. C. A few weeks ago. 3. What did the students find when they came to the river this year? A. There was no garbage left to clean up. B. The river had become to clean that a lot of water-birds came back. C. The river was much cleaner and they had to search for garbage. 4. What was the expected reaction of the local people into the river. A.Most of them would keep on throwing garbage into the river. B.They would join the students in changing the situation. C.They would become more aware of the pollution problem.