A、How the Great Canyon came into being. B、The fantastic views in the Great Canyon. C、The wonderful trip to Arizona. D、Colorado R

M: It’s good to see you back. How was your trip to Arizona? Did you see the Great Canyon?
W: Yes, it was fantastic. Now I know why it is listed together with the Nile River as one of the greatest natural wonders in the world. Here, do you have interest to see the pictures I took there?
M: Oh, what a splendid view! It is even bigger than I had imagined. I remember reading about the Great Canyon and(9)I think it said that it was formed suddenly when the outer crust of the earth split open during an earthquake.
W:(9)Well, not that quickly. Look, here is the picture of the Colorado River. See the way down at the bottom of the canyon? That river and its branches have been wearing the canyon floor away for ten million years.
M:(10)Then the canyon is the result of soil losses. I’ll bet geologists have made some interesting discoveries there.
W: Not only the geologists, archaeologists have also found the bones of some extinct animals in caves in the canyon walls. In a cave 140 feet above the river, they found driftwood that dated back as far as 37 000 years.
M:(ll)That means the river must have been 140 feet higher when it carried the driftwood into the cave.
W: It’s very possible, but of course who knows. The Great Canyon is full of mysteries. Wouldn’t you like to go to Arizona some day?
M: You bet I would.
9. What are the two speakers mainly talking about?
10. What’s the main cause of the Great Canyon’s formation according to the conversation?
11. What does the driftwood found in the cave indicate?