Physician: In an experiment, 50 patients with chronic back pain were divided into two groups. Small magnets were applied to the backs of one group; the other group received no treatment. Most of the patients in the first group, but very few in the second group, reported a significant reduction in pain. This shows that magnetic fields are probably effective at relieving some back pain. Which one of the following, if true, constitutes the logically strongest counter to the physician's argument? ['There was wide variation in the specific causes of the chronic back pain suffered by the patients in the experiment.', "A patient's merely knowing that a treatment has been applied can lead to improvement in his or her condition.", 'Most physicians believe that medication relieves chronic back pain more effectively than magnets do.', 'No other experiments have been done showing that magnetic fields reduce pain in any area other than the back.']