A、It is difficult to obtain. B、Paper cannot be made without it. C、It causes paper to deteriorate. D、It prevents wood from being

Moderator:
    Good evening, everyone! It’s a great honor of us to have Professor Johnson here. As we all know, Professor Johnson is an expert of the paper industry. Today he will talk about the development of paper. Let’s welcome Professor Johnson.
Professor Johnson:
    Thank you for the introduction. Ok, now I begin the lecture.
    In the early 1800s, the paper industry was still using rags as its basic source of fiber as it had for many centuries. However, the rag supply couldn’t keep up with the growing demand for paper.(16)The United States alone was using 250 thousand tons of rags each year. And a quarter of that had to be imported. It was clear that a new source of fiber was needed to keep up with the demand for paper. The answer to this problem turned out to be paper made from wood pulp, something that was abundantly available in north America.(17)In Canada, the first wood pulp mill was set up in 1866 and it was immediately successful. But while wood pulp solved the problem of quantity, it created a problem of quality. Wood contains a substance called lignin. The simplest way to make large quantities of cheap paper involves leaving the lignin in the wood pulp.(18)But lignin is acidic and its presence in paper has shorten the life expectancy of paper from several centuries for rag paper to less than a century for paper made from wood pulp. This means that books printed less than a hundred years ago are already turning yellow and beginning to disintegrate, even though books printed much earlier may be in fine condition.(19)This is bad enough for the older books on your bookshelf but it poses a huge problem for libraries and the collections of government documents.
16. Why did the paper industry need a new source of fiber in the early 1800s?
17. What can be inferred about the first wood pulp mill in Canada?
18. According to the speaker, what is the problem with lignin?
19. According to the speaker, what problem do libraries face?