A teacher wants to examine the impact of caffeine on test performance. She randomly assigns 60 students in her class to three groups (no caffeine-placebo, 100mg of caffeine, 200mg of caffeine) and then 40 minutes later students takes a test. Which would be her independent variable
a. caffeine level
b. no caffeine vs test performance
c. test performance
d. the 40 minute time period
e. 60 students