听力原文：W: Excuse me, I've been using this old book for a research project and I noticed that a lot of pages are turning brown and becoming brittle. M: Yes, unfortunately, that's a common problem with books made from wood pulp. W: I suppose to make paper from wood that you have to add a lot of chemicals and acids to make it turn white. M: Exactly and it's the acid that eventually eats away the paper. W: Oh, that actually make sense, but this book's not even 75 years old and I've seen books in museum that are hundreds of years old and they're in fine condition. M: Well, you see, books have been made from wood pulp only since the 1850's, before that they were mostly made from materials like animals' skins, and no chemicals were added. W: It's a shame those older wood pulp books are going to fall apart some day. Is there anything that can be clone to preserve them? M: Eh, currently the only way to stop the books from decaying is to remove the binding and treat each page individually to remove the acid. W: That doesn't sound very economical. M: No, it isn't. It's not practical to treat a large number of books with this process, so we only try to rescue the most valuable 1st edition books in our collection. W: Well, thanks for the explanation. I'd better get back to my project. M: Good luck, and I hope the old book holds long enough for you to finish it. (20)