听力原文：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.
Yes, unfortunately, that's a common problem with books made from wood pulp.
I suppose to make paper from wood that you have to add a lot of chemicals and acids to make it turn white.
Exactly and it's the acid that eventually eats away the paper.
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.
Well, you see, books have been made from wood pulp only since 1850's, before that they were made from materials mostly animals' skins or C, no chemicals were added.
It's a shame those older wood pulp books are going to fall apart some day. Is there anything that can be done to preserve them.
En, currently the only way to stop the books from decaying is to remove the binding and treat each page individually to remove the acid.
That doesn't sound very economical.
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.
Well, thanks for the explanation. I'd better get back to my project.
Good luck, and I hope the old book hold long enough for you to finish it.
(31)

A. The early history of bookbinding.
B. How old books become valuable.
C. Economical ways to protect old books,
D. Why some books deteriorate.