Mr. Doe wanted to perform an experiment to determine if doing a class review before a test would increase test scores. Mr. Doe normally does not review. To perform this experiment, Mr. Doe reviewed material with 2nd period before a test and did not review with 3rd period. After grading papers, he found that 2nd period had an average grade of 93 and 3rd period had an average grade of 72. He determined that reviewing before a test might increase test scores. 2. Which of the following is the control group?