It's not a well-written question. As written, the answer is clearly E; the integers can be negative (they don't say in the question that the integers are positive). Even if you look only at positive integers, the answer is E anyway, but it's a bit tedious to see why. I posted a solution here:

www.beatthegmat.com/please-help-me-t17777.html

It's a more interesting problem if you disallow the set 6, 7, 8, ..., 17, 18, and I solved that version of the problem in the above post as well. Regardless, I don't find the question all that similar to real GMAT questions, so it isn't worth spending much time on.