Please cite the source (author) of this problem. We cannot reply unless a source is cited (and, if no source is cited, we will have to delete the post!). Thanks.
Please also make sure to read (and follow!) the forum guidelines before posting. The subject heading should be the first 5-8 words of the problem.
I wish we could help you, but we can't. The reason we ask for sources was there have been times in the past where people took questions from the actual GMAT and would discuss them live. So unless we have a source, we can't even give you a hint about the answer.
At this point, even if that was an experimental question, it's shelf life on the GMAT has been used up, but we have to be cautious. Sorry :)