听力题 Last August, Susan and 42 other students got wet and dirty while removing six tons of garbage from the river running across their city. They cleaned up the river as part of a week-long environmental camp. Like one in three American rivers, this river is so polluted that it''s unsafe for swimming and fishing. Still, according to Susan, who has just completed her third summer on the river clean-up, the scene has changed. "Since we started three years ago, the river is getting a lot cleaner," she says. Environmental scientists praised the teenagers for removing garbage that can harm wildlife. Water birds, for example, can choke on plastic bottle rings and get cut by scrap metal. Three years ago when the clean-up started, garbage was everywhere, but this year the teenagers had to hunt for garbage. They turned the clean-up into a competition to see who could find the most garbage and unload their boats fastest. By the end of the six hour shift, they had removed enough garbage to fill more than two large trucks. "''Seeing all their garbage in the river makes people begin to care about environmental issues," Susan says. She hopes that when others read that she and her peers care enough to clean it up, maybe they will think twice before they throw garbage into the river.