A、The book has been returned on September 27. B、The book has been sold out on September 27. C、The book has been checked out on S

M: Hello.
W: Hello.(19)I wonder if you could help me. I received this note yesterday.
M: Let’s see.(19)Yes. It’s 15 pounds for a lost book.
W:(20)But I didn’t lose the book. I’m quite sure I returned it.
M:(20)Let me check. Just a minute.(21)Our record shows that you checked it out on September 27 and we have no record of its being returned.
W: I’m sure I returned it. I remember that it was a rainy day and I had to carry it in a plastic bag.
M: Well, our policy is that, when a book is lost, the borrower must pay for it. We have to be fair about these things.
W: I understand your position. Is there a way we can check to see if the book is here, just in case someone missed it?
M: Well, I don’t think we have made a mistake. But you can look for it on the shelf.(Later...)
W: Here it is.
M: Let me see if it’s the right edition. I’ll check the number on it with our records.
W: I’m sorry to bother you with this.
M: Well, we could have missed something. Yes, it is the right edition. You are right. I’m awfully sorry.
W: That’s all right.
M:(22)Sometimes we get in a hurry and forget to check some books in. That must be how it has happened. I’m very sorry.
W: Don’t worry about that.
M: Thanks for coming in.
19. What is the woman doing at the beginning of the conversation?
20. Where does the conversation most probably take place?
21. What do we learn from the library’s record?
22. According to the man, what’s the most probable reason for the mistake?