听力原文：W: I haven't seen you for a while. Have you been away? M: Yes, I just got back from vacation. I took a tour around the Great Lakes. Have you ever been there? W: 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. M: But I thought that Lake Erie is so polluted that everything living in it had died. W: At one time Lake Erie was almost dead. There was so much algae that the oxygen supply was practically used up. M: It was the amount of phosphate in detergents that caused the excessive growth of algae, wasn't it? W: Yes, but since both the U. S. and Canada have now limited phosphate in detergents, there have been tremendous gains in the fight against water pollution. In fact, all the Great Lakes will be ninety percent improved within the next generation. M: I'm glad to hear an environmental success story at last! What's the main subject of this conversation?