Researchers Were Interested in Whether Relaxation Training Decreases the Number

Researchers were interested in whether relaxation training decreases the number of headaches a person experiences.They randomly assigned 20 participants to a control group (no relaxation 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) training type.
B) number of headaches.
C) control group.
D) relaxation training.