This question highlights the exact type of logic that I have struggled with correcting in my head. It's considered an 'easy' question, and so I thought I'd contribute a post here for both my own learning. Also, I often times find that just reading different perspectives on here helps me get a better grasp on a question and it's respective solution, so I'm trying to do my part =)
I was quickly able to widdle this problem down to two possible options (C) and (E). Answer choice (C) sounded awful on point; however, I was really attracted to option (E). I almost treated it as a fill in the blank with the spirit of "fill in the next step forward". With that, I thought "well if a signature is so hard to forge because the machine scrutinizing it is also measuring things like pressure/speed etc., then it seems very possible that authorized people might get rejected like a false positive.
I let this problem marinade in my head throughout my day, and as I read and re-read our resident expert responses, I think I'm slowly coming to the 'ah hah' moment that I need to have. If you had difficulty between options (C) and (E), maybe this will help. For me, I had to realize, yet again, the prompt states
which of the following can be logically concluded from the above. The word 'above' in the prompt is quite literal, and the logical conclusion would be that since men the most adept forgers can't duplicate all of the characteristics, nobody can gain access to a computer equipped with the software solely by virtue of their forgery skills. By this logic, I'm not bringing anything new into the passage.
However, if we are looking at answer choice (E), 'authorized users' are brought up and this population is not addressed anywhere in the passage. What made this choice appeal to me though was that I conflated 'authorized users' with 'signatures on file'. As soon as I realized that I did this, the correct answer, (C), became very obvious to me.
Please feel free to let me know if anything was confusing. Hopefully I did more good than harm but contributing to this discussion. Thanks everyone for posting!
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(C) Nobody can gain access to a computer equipped with the software solely by virtue of skill at forging signatures.
(E) In many cases even authorized users are denied legitimate access to computers equipped with the software.