Sorry folks, wasn't able to find this one in the forum already so hope I'm not duplicating. Source is GMATPrep, I got this as a question in Exam Pack 1.
Exam pack 1 is a paid for resource and is copyright material of GMAC. Please don't post questions from it! You may be banned from the forums if you do so again. Please read the guidelines before posting.
I recommend that you ask your course tutor about the question.