A、If a student has written a note in it. B、If it was purchased at another store. C、If a professor decides to use another text. D

W: That comes to $ 160.
M: A hundred and sixty dollars! I just can’t believe how expensive textbooks are. And that’s just for required texts. (19)Why, if I had to buy all the books on my suggested reading lists, I’d have to take out a bank loan!
W: You could save some money if you bought used texts, you know.
M: I suppose, but it’s hard for me to study from a text that’s been marked up. Tell me, if I don’t need some of these books, can I get a full refund?
W: Sure, if the professor changes his mind about a book or if you drop a course, just return it and we’ll give you your money back — but only for the first three weeks of classes. So don’t write your name in the textbook or mark it up until you’re sure you’re going to keep it all semester.
M: And what about at the end of the semester? What’s your buy-back policy? (20)
W: As long as the books are in reasonably good condition, and they’re going to be used in class the next semester, we’ll give you 50 percent of their original value (20) — even if you didn’t buy them at the store. Of course, if a professor changes texts or if a new edition conies out, we won’t buy them back at all. (21)
M: Fifty percent — that’s all?
W: Well. I suppose that doesn’t sound like much, but that’s the store policy.
19. What is the man buying?
20. If the man sells all the books that he buys today back to the store at the end of the semester, how much money will he receive?
21. Why would the bookstore NOT buy back the man’s books at any price?