A、In a boat. B、In a cave. C、In a river. D、In a tree. B

M: It’s good to see you back. How was your trip to Arizona? Did you see the Grand Canyon?
W: Yes, it was fantastic. Now I know why it is listed together with the Nile River as one of the great natural wonders of the world. Here, want to see the picture I took?
M: Oh, what a view! It is even bigger than I had imagined. I remember reading about the Grand Canyon and I think it said that it was formed suddenly when the earth’s crust split open during an earthquake.
W: Well, not that quickly. Look, here is the picture of Colorado river. See way down at the bottom of the canyon, that fiver and its tributaries have been wearing the canyon floor away for ten million years.
M: Then the canyon is the result of swale erosion. I’ll bet geologists have made some interesting discoveries here.
W: Not only the geologists, archaeologists have found the bones of extinct animals in caves in the canyon wails. In a cave one hundred and forty feet above the river, they found drift wood than dates back as far as thirty seven thousand years.
M: That means the river must have been a hundred and forty feet higher up when it carried the driftwood into the cave.
W: It’s very possible, but of course who knows. The Grand Canyon is full of mysteries. Wouldn’t you like to go to Arizona some day?
M: You bet I would.