A、They are marked with colored labels. B、They are specially ceded. C、They are checked out. D、They are inspected by tile guard. B

M: Do you know what happened to me today? I was so embarrassed.
W: What?
M: Well, Doctor Bill’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 industrial revolution. I looked them up in the library and found them. So I put them in my book-bag and headed back toward the door. Then it happened.
W: What did it happen?
M: 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. Then why didn’t the exit gate open?
M: I asked him that. It seems that the books in the library are all specially ceded and when you check them out, the librarian behind the desk helps you decode 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.