A、They are checked out. B、They are specially coded. C、They are inspected by the guard. D、They are marked with colored labels. B细

M: Do you know what happened to me today? I was so embarrassed.
W: What?
M: Well, Dr. Brown’s class finished ten minutes early, so I went to the library between classes. I knew I didn’t have much time, but I wanted to get those books on the Industrial Revolution. I looked them up in the card catalog and went right to the stacks and found them. So t put them in my book bag and headed back toward the door. Then it happened. The exit gate in front of the door wouldn’t open, and a guard immediately, warned me that I hadn’t checked out my books. He thought I was trying to steal them.
W: That must have been embarrassing. But why didn’t the exit gate open?
M: I asked them that. It seems the books in the library are all magnetically coded, and when you check them out, the librarian behind the desk demagnetizes them, then the exit gate will open.
W: How interesting! I still don’t quite understand how they do it, though. I’ll have to go to the library and see it for myself.