听力原文：M: Hey, Sue. I was wondering if you could fill me in on Monday's class. I had to go the dentist's for an emergency and I missed Prof. Smith's lecture(19). What was it on? W: It was pretty interesting. She talked about volcanoes, active volcanoes, under the---uh-- West Antarctic ice sheet(20). M: There are active volcanoes under the ice? W: Apparently so. She said they help protect the ice sheet and prevent melting. Flooding would be pretty bad if that ice melted, not only there, but all over the world. M: You lost me there. Volcanoes are hot. How can something hot prevent ice from melting (21). W: Wait a minute, let me check my notes. Yeah, here it is. Volcanic heat melts just enough ice to create a slippery surface on the bottom of the glacier. This water allows ice to flow out into the ocean. So the solid interior ice is protected from the ocean's warmth. Does that make sense? M: Sort of. You mean that because the ice is flowing out to the ocean, the warmer ocean water can't flow in. W: Exactly. And the ice that melts is constantly being replaced by snow. Prof. Smith said that if the ice sheet ever broke up and melted, the sea level would go up seven meters, Then we would have those floods. M: Is that really possible? Or is it one of those exaggerations you hear all the time? W: As far as I can understand, it is possible, because of global warming. I mean if the ocean got a lot warmer that interior ice would be very likely to melt. M: Thanks for telling me about the lecture. Sounds like I've missed a pretty important class. (23)