A professor is making a new strategy to help students preform better on exams. He randomly selected two groups of his students. One group gets review, and the other group doesn't. He then compares student exam agrores between those who participated in the review and those who did not. He predicts that the students who were in the review preformed better than those who were not in the review. What is the explanatory variable for the experiment? A. The prediction that exam scores will go up. B. Review sessions before exams. C. His strategy will help his students. D. Students exam scores