A、In a boat. B、In a cave. C、In a river. D、In a tree. B女生的一句话讲到这一点：In a cave one hundred and forty feet above the river，they foun

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 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’ 11 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 walls. In a cave one hundred and forty feet above the river, they found drift wood that 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.