Dr. Pena is conducting research to see whether the severity of the chronic headaches suffered by her patients will be lessened through the use of biofeedback. Based on what she has read in medical journals and on her own observations, Dr. Pena has determined that the headaches are caused by muscle tension, so she feels that patients who learn to relax will have fewer or less severe headaches. To test her theory she attaches electrodes to the forehead of each of her patients. These electrodes measure electrical activity from the subjects forehead muscles and drop when they receive feedback in the form of soothing music. Dr. Pena finds that compared to a separate control group of people to whom electrodes are attached but do not receive the feedback, the experimental group reports fewer and less severe headaches during the week following the biofeedback session.

Dr. Pena used a _______________________ -subjects design in her experiment.
a. Within
b. Between
c. Mixed
d. Crossed
e. Mixed-Within