An experimenter studies the relationship between caffeine and reaction time. she designs her experiment with four groups. group 1 receives 100 mg of caffeine each in their cups of coffee, group 2 receives 200 mg of caffeine each in their cups of coffee, group 3 receives 300 mg of caffeine each in their cups of coffee, and group 4 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?
a. group 4
b. group 3
c. group 2
d. group 1