听力原文：M: How did you like yesterday's astronomy class? W: It was interesting. But the point she was trying to make seemed a little far-fetched. M: Oh, that new theory that ocean water came from comets? W: Yeah. Do you remember what it was based on? M: Some recent satellite photos, I think. Apparently, space satellites recently detected thousands of small comets colliding with earth's outer atmosphere, almost 40,000 per day. W: Ok, so they collided with the atmosphere. So that's what created the water? M: It's not that the collision created water. Comets contain water. They are made up mostly of cosmic dust and water. When they collide with the atmosphere, they break up. And the water they contain rains down to earth. Ocean water came from that rain. W: Oh. Well, this morning I asked my geology professor about that. He said that most geologists draft accept it. M: Why not? W: Their research indicates that most of the water molecules from the comet would have burned up as they fell through the atmosphere. Enough rain couldn't have reached the earth to fill up the ocean. M: Well, did the geologists have an alternative theory to explain where ocean water came from? W: Yeah, he said the more traditional view is that the ocean water came from volcanoes. M: From volcanoes? W: Right. They say volcanic fumes are mostly steam. And they claimed that it was the volcanic steam that created the oceans, not rain from comets. What are speakers mainly discussing?