Question: A class is testing the effect of exercise on heart rate. The heart rate of two students before exercise is compared to their heart rate after running around a track. Which procedure will most likely help the class correctly compare the results of the two students?

  Options: 
 A. test students who are the same height
 B. have the students run on the same day
 C. measure the distance each student runs
 D. have the students run for the same amount of time
Answer: D