My answer is C. I only had to choose between C and E.
I eliminated A,B and D because there was no comma before it.
Here is a tip from Erins' test magic forum.:
But first, a testmagic tip: GMAT almost always (I say almost always because I've seen two questions that did not follow this rule, but the rule was violated in all five answer choices) wants you to put a comma before which. In other words, if you see which without a comma before it, it's probably wrong.
510 on my first GMAT. 610 on second GMAT.!
The struggle continues.