M: Some recent satellite photos, I think. Apparently, space satellites recently detected thousands of small comets colliding with earths outer atmosphere, almost 40,000 per day.

　　W: OK, theyre collided with atmosphere. So thats what created the water?

　　M: Its not the collision that created water. Comets contain water. Theyre 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 dont accept it.

　　M: Why not?

　　W: Special research indicates that most of the water molecules from the comet would have burned up as they fell through the atmosphere. Enough rain couldnt 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.

　　Questions 23 to 25 are based on the conversation you have just heard.

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