A Professor Was Interested in Whether Caffeine Affected Test Performance

Multiple Choice

A professor was interested in whether caffeine affected test performance. To test this possibility, he gave students a test and asked them whether they had consumed caffeine in the hour before the test. He then compared the performance of those students who had taken caffeine to those who had not. Why is this not a true experiment?

A) There are not two levels of the independent variables.
B) Participants were not randomly assigned to groups.
C) The study was not performed in the laboratory.
D) The study lacks external validity.