Questions 4 to 6 are based on the following conversation.
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's based on?
M: Some recent satellite photos, I think. Apparently, [4] space satellites recently detected thousands of small comets colliding with earth's outer atmosphere, almost 40,000 per day.
W: Ok, so they collide with the atmosphere. Then 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 wamr they contain rains down to earth.Ocean water came from that rain.
W：0h，well，this morning，I asked my geology professor about that.He said that most geologists don’t accept it.
M：Why not?
W：Well,[5]their research indicates that most of the water molecules from the comets will have burnt up a_As they fell through the atmosphere.Enough rain couldn't have reached the earth to fill up the ocean.
M：Well.[6] do the geologists have an alternative theory,to explain where ocean water came from?
W：Yeah,[6]he said the more traditional view is that ocean water came from volcanoes.
M：From volcanoes?
W：Right! As the volcanic fumes are mosdy steam.And they claim that it was volcanic steam that created the oceans，not rain from comets.
Keys：4.[B]5.[D]6.[C]