An experimenter wants to study the relationship between caffeine and reaction time. She assigns participants as follows: Group One receives 50 milligrams of caffeine; Group Two receives 100 milligrams of caffeine; Group Three receives 200 milligrams of caffeine; Group Four receives no caffeine. Twenty minutes later, all participants complete the reaction-time test. In this experiment, what was the dependent variable?

a. Amount of caffeine
b. No caffeine
c. Reaction time
d. Assignment into one of four groups