Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now, listen to the passage. What did the students find when they came to the river this year
A．There was no garbage left to clean up. B．There was more garbage than before and they had to work harder. C．The river had become so clean that a lot of water birds came back. D．The river was much cleaner and they had to search for garbage.