In an experiment, some volunteers were assigned to take aerobics classes and others to take weight-training classes. After three months, each performed an arduous mathematical calculation. Just after that challenge, the measurable stress symptoms of the volunteers in the aerobics classes were less than those of the volunteers in the weight-training classes. This provides good evidence that aerobic exercise helps the body handle psychological stress. Which one of the following is an assumption the argument requires? ['The volunteers who were assigned to the aerobics classes did not also lift weights outside the classes.', 'Three months is enough time for the body to fully benefit from aerobic exercise.', 'On average, the volunteers who were assigned to the aerobics classes got more exercise in the months in which they took those classes than they had been getting before beginning the experiment.', 'On average, the volunteers assigned to the aerobics classes got a greater amount of aerobic exercise overall during the experiment, including any exercise outside the classes, than did the volunteers assigned to the weight-training classes.']