In an experiment testing whether hyperactivity is due to a brain abnormality, the brain activity of 25 hyperactive adults was compared to the brain activity of 25 adults who were not hyperactive. The tests revealed that the hyperactive adults had much less brain activity in the premotor cortex, a region of the brain believed to control action, than did the nonhyperactive adults. The experimenters concluded that diminished activity in the premotor cortex is one cause of hyperactivity. Which one of the following, if true, most undermines the conclusion drawn by the experimenters? ['All of the hyperactive adults in the study had been treated for hyperactivity with a medication that is known to depress activity in some regions of the brain, while none of the nonhyperactive adults had been so treated.', 'The test was performed only on adults because even though the method by which the test measured brain activity is harmless to adults, it does require the use of radiation, which could be harmful to children.', 'Some of the nonhyperactive adults in the study had children who suffer from hyperactivity.', 'The neuropsychologists who designed the experiment were not present when the tests were performed.']