In a recent study, one group of participants watched video recordings of themselves running on treadmills, and a second group watched recordings of other people running on treadmills. When contacted later, participants in the first group reported exercising, on average, 1 hour longer each day than did the other participants. This shows that watching a recording of yourself exercising can motivate you to exercise more. Which one of the following, if true, most weakens the argument? ['In studies of identical twins, participants who observed their twin reading overreported by a significant amount how much time they themselves spent reading in the days that followed.', "Another study's members exhibited an increased willingness to give to charity after hearing stories in which people with whom they identified did so.", 'Participants who were already highly motivated to exercise did not report exercising for any longer each day than they had before the study.', 'In another study, people who watched recordings of themselves lifting weights exercised for more time each day than did people who watched recordings of themselves running.']