A professor of business placed a case-study assignment for her class on her university' s computer network. She later found out that instead of reading the assignment on the computer screen, 50 out of the 70 students printed it out on paper. Thus, it is not the case that books delivered via computer will make printed books obsolete. Which one of the following, if true, most strengthens the argument? ['Some people get impaired vision from long periods of reading printed matter on computer screens, even if they use high quality computer screens.', 'Studies consistently show that most computer users will print reading material that is more than a few pages in length rather than read it on the computer screen.', 'Books on cassette tape have only a small fraction of the sales of printed versions of the same books, though sales of videos of books that have been turned into movies remain strong.', 'Scanning technology is very poor, causing books delivered via computer to be full of errors unless editors carefully read the scanned versions.']