Salvador wants to test the effects of creatine on runners' performance during a race. Half of the runners will have a drink that contains creatine and the other half will have a drink that does not contain creatine. The runners will then race and will be timed with a stopwatch. In Salvador's study, which of the following is the dependent variable?
a. the runners' performance
b. the drink they receive
c. the stopwatch
d. the type of race they run
