A、Those without both covers. B、Those over 75 years long. C、Those with only one edition. D、Those worthwhile ones. D

W: Excuse me,(12)I’ve been using this old book for a research project and I noticed that a lot of pages are turning brown and becoming delicate.
M: Yes, unfortunately, that’s a common problem with books made from wood.
W: I suppose to make paper from wood, 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 makes 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,(13)books have been made from wood only since 1850’s. Before that they were made from materials mostly animals’ skins or cloth based, no chemicals were added.
W: It’s a shame those older books made from wood are going to fall apart some day. Is there anything that can be done to preserve them?
M: En,(14)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.(15)It’s not practical to treat a large number of books with this process, so we only try to rescue the most valuable first 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 books hold long enough for you to finish it.