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, 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 was so polluted that almost everything living in it had dieD. W: At one time Lake Erie was almost deaD.There was so many algae that the oxygen supply was practically used up. M: It was the amount of phosphates and detergents that caused this excessive growth of algae, wasn't it? W: Yes, but since both the United States and Canada have now limited phosphates and detergents, there has been tremendous gain in the fight against water pollution. In fact, an the Great Lakes will be ninety percent improved within the next generation. M: I' m glad to hear an environmental success story at last. This conversation is mainly about ______ A.United States-agreement B.Unlimited growth in population C.Pollution in the Great Lakes D.The limited supply of phosphates
A.M: B. M: C. W: D. M: E. M: F. This G.United H.Unlimited I.Pollution J.The