听力原文：M: I haven't seen you for a while. Have you been away? W: Yes, I just got back from vacation. I took a tour around the Great Lakes. M: Was that an exciting experience? W: Sure. Have you ever been there? M: Oh, yes! When I was young I lived in Erie county in New York and I used to go swimming in Lake Erie all the time. We also went to Lake Ontario in Canada. W: But I heard that Lake Erie was so polluted that almost everything living in it had died. M: At one time Lake Erie was almost dead. There was so much algae that the oxygen supply was practically used up. W: What on earth caused algae to grow so much? M: It was the amount of phosphate detergents that caused the excessive growth of algae. W: Oh, I think it's the time that we limit the use of phosphate detergents. M: Yes, and both U.S. and Canada now have legally limited the use of phosphate detergents, there have been tremendous gains in the fight against water pollution. W: That's good. M: In fact all the Great Lakes will be ninety percent improved within the next generation. W: I'm glad to hear an environmental success story at last! (23)