During the period in which there are no competitive races, two runners--Runners A and Runners B--take part in an experiment measuring their VO2 max, the volume of oxygen an athlete can use. During these sessions, the runners engaged in moderate aerobic activity, or a sustained heart rate between 146-154 beats per minute. At the end of the sessions, Runner A had a greater VO2 max than did Runner B. Ttherefore, once the two runners begin identical intensive training--sessions involving over 168 beats per minute-- for the race season, Runner A will continue to have the greater VO2 max, assuming that neither become injured and that both train with similar intensity. Which of the following is an assumption upon which the argument rests? ['Runner A and Runner B had similar VO2 maxes upon entering the study.', "The amount one trains does not influence one's VO2 max.", 'Regarding their VO2 maxes, runners respond equally to intensive training.', 'Intensive training involves sessions in which athletes maintain a heartbeat over 168 beats per minute.']