An experimenter studies the relationship between caffeine and reaction time. she designs her experiment with four groups. group one receives 100 mg of caffeine each in their cups of coffee; group two receives 200 mg of caffeine each in their cups of coffee; group three receives 300 mg of caffeine each in their cups of coffee; and group four receives no coffee. twenty minutes later, participants from all the groups are given a reaction-time test. in this experiment, which group is the control group?