An investigator conducts an experiment to determine if exercise improves the cognitive processing speed of older adults. He randomly selects two equivalent groups; one of the groups exercises, the other does not. He compares the average processing time of the exercise group to the no exercise group to evaluate his research hypothesis. What is the Dependent Variable in this experiment? a. The average processing times of participants in the two groups. b. The absence of exercise. c. The presence of exercise. d. Both the presence and absence of exercise