Arthur reads an article describing an experiment to test the effects of caffeine on the reaction time of humans—how long it took them to respond to a particular signal. in the experiment, the subjects' reaction times were measured, then they were divided into three groups. one group was given a high dose of caffeine. the second group was given a medium dose of caffeine. the third group was given a sugar pill that had no caffeine. after waiting 45 min for the caffeine to take effect, the reaction times were measured again.

in the experiment arthur read about, what was the dependent variable?
a.
the amount of caffeine given to each person
b.
the type of signal each person responded to
c.
the amount of time the caffeine was given to take effect
d.
the change in reaction times for each person