A researcher plans to study the effect of cardio exercise versus weight lifting on motivation. She randomly assigns half the participants to attend a weight-lifting class at the gym next door to the lab twice a week in 45-minute blocks and the other half to attend a cardio class at the same gym three times a week in 30-minute blocks. She then measures motivation each week over a 2-month period. A possible confound in this study is a. the type of exercise. b. the difference in gyms. c. motivation. d. the time spent in the gym