Researchers were interested in whether relaxation training works better than fitness training in decreasing the number of headaches a person experiences. They randomly assigned 20 participants either to the fitness training group or a relaxation training group and then recorded the change in number of headaches for each group from pre-training to post-training. The independent variable in this study is ___.

A. change in number of headaches

B. fitness training

C. relaxation training

D. training type