Remember, the question stem is asking us to determine why the argument is most vulnerable to criticism.
I agree that answer D seems right insofar as it could potentially negate the salesperson’s overall conclusion: that the Super XL is the better vacuum. However, we need to find the flaw in the salesperson’s /argument/, and merely describing a possible reason that the old vacuum is better does not describe a flaw in the argument.
Answer E does describe the flaw. It is crucial to recognize that the salesperson’s argument was linked to the order of vacuuming: first the old vacuum, then the XL. The XL picked up dirt left behind, and therefore (the salesperson claims) it is better. Answer E points out that this argument is flawed insofar as we don’t know if the same thing would have happened in reverse if the order had been reversed as well, and thus the conclusion is not valid.
I hope this helps! Let us know if you have further questions.
