Researchers were interested in whether mindfulness meditation training decreases the number of headaches a person experiences. They randomly assigned 28 participants to a control group (no mindfulness meditation training) or a relaxation training group and noted the change in number of headaches each group reported from the week before training to the week after training. The independent variable in this study is: a. number of headaches. b. training type. c. relaxation training. d. control group.