A、Somebody else has bought it. B、The assistant has put it in a corner. C、It has been stolen. D、It is being read by someone else.

We often feel embarrassed if we enter a shop and don’t buy anything. But this doesn’t happen if we enter a good bookstore. Time spent in a bookshop can be enjoyable if you go to a good shop. No assistant will come near to you and say "Can I help you?" and you needn’t buy anything you don’t want. You may try to find out where the book you want is.(11)The assistant will lead you there and then he will go away and look as if he is not interested in selling a single book. There is a story which tells us about a good shop. A medical student has to read a textbook which is too expensive for him to buy. He can’t get it from a bookshop.(12)So every afternoon he goes there to read a little at a time. One day, however, he can’t find it in its usual place and is about to leave when he notices an assistant signing to him. To his surprise, the assistant points to the book, which is in a corner.(13)"I put it there so as not to sell it," says the assistant. Then he leaves the student to continue his reading.
11. What does the speaker think an assistant should do in a good bookstore?
12. When does the medical student usually go to the bookshop?
13. Why can’t the student find the book in its usual place?