A track and field coach wants to analyze the effect of sports drinks on the performance of his athletes. He decides to test three brands: Powerade, Gatorade and Vitaminwater. He will have each of the athletes complete a 400 meter run and record their times. Which of the following represents a confounding variable? Mark all that apply.

a. He has some of the athletes drink Vitaminwater and run in the morning, some drink Gatorade and run midday and some drink Powerade and run in the afternoon.
b. He allows each athlete to choose their favorite drink from a cooler.
c. He has all his athletes repeat the 400m on different days after having one of the three drinks- on Monday they all have Gatorade, on Wednesday Powerade and on Friday Vitaminwater.
d. He has some of the athletes run in the morning, some midday and some in the afternoon.
e. He has some of the athletes drink Gatorade two hours before running, others drink Powerade one hour before running, and the rest Vitaminwater half an hour before running.
f. He decides to give all male athletes Gatorade and the female athletes either Powerade or Vitaminwater.
g. He has some of the athletes drink the sports drink two hours before running, others one hour before running, and the rest 30 minutes before running..