A football coach hears that a new exercise program will increase upper-body strength better than lifting weights. He is eager to test this new program in the off-season with the players on his high school team The coach decides to let his players choose which of the two treatments they will undergo for 3 weeks-exercise or weight lifting. He will use the number of push-ups a player can do at the end of the experiment as the response variable. Which principle of experimental design does the coach's plan violate? Random assignment Replication Control Comparison