I found this question at Gmat club.http://gmatclub.com/forum/the-sunrise-p ... 76556.html
Dont know the original source. I was not convinced with the answers posted there. So looking for some more explanation on this.
Unfortunately we are required to obtain original sources for all problems we tackle; we'll be unable to help further unless you can obtain the original source for this problem. Otherwise, we run the risk that this is actually an official GMAT problem, and those are banned on this site. Sorry. :-(