Elias conducted an experiment about the effects of exercise on the heart rate of teenagers. Before morning basketball practice, he asked the members of his high school team to be his subjects. He measured each of their heart rates before and after a set of exercises. Which change would most reduce the bias of his experimental design?
A. Test the team members on the same exercises after practice too.
B. Test additional boys as well as several girls from different classes.
C. Test girls from his school, as well as teachers, siblings, and family friends.
D. Test the team members every day for a week to increase data.