This is a tough question and there are many answer choices that at first sound very good. I agree that answer choices (A), (B), (C), and (D) all warrant at least a bit of consideration. This is a good example to practice moving from wrong to right. The key here would be to look for words that would allow us to eliminate the answer choice.
We're asked to strengthen the argument that concludes that it is not the case that books delivered via computer will make printed books obsolete. This is a fairly wide ranging statement, so answer choices about "some" people's behavior will not suffice.
The premise is an example of students printing out an assignment. What's the gap? Well, perhaps that's just a rare example. Also, that example is about assignments, while the conclusion is about books. Does this evidence fit into a wider body of evidence that includes books?
(A) can be eliminated because of the word "several." It's simply not strong enough to support the general conclusion reached in the stimulus. Plus, the conclusion is about books, and we need to be able to link the information about assignments to books.
(B) provides a statement about "most" computer users and so is strong enough to support the general conclusion reached in the stimulus. Also, it broadens the evidence to include "reading material" which includes books.
(C) can be eliminated because it says "some people." Again, it's not strong enough to support the general conclusion that is reached.
(D) provides an opt out clause. It says that books delivered via computer are full of errors unless editors carefully read the scanned versions. Well, we don't know that editors are not carefully reading the scanned versions, so we don't know whether books delivered via computer are actually full of errors.
(E) is irrelevant.
When you have so many good looking answer choices, it's best to start being very detail oriented. Focus on the specifics of the language, because the big picture of the answer choice already passed your first pass through. Let me know if you still need more help with this one!
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