听力原文： Last August, Susan and 42 other students got wet and dirty while removing six to

ns 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 its usage for swimming and fishing has long lost. Susan, who has just completed her third summer on the river clean-up, tried to bring changes to the river. "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 wild life. 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 faster. By the end of the six-hour shift, they have 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 in the river.

What does the passage tell us about American rivers？

A．One sixth of them are seriously polluted.

B．One third of them are seriously polluted.

C．Half of them are seriously polluted.

D．Most of them are seriously polluted.