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?

Pick the answer from these options

Possible answers:
(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;
(D).