A Music Professor Offers His 40 Students the Option of Coming

A music professor offers his 40 students the option of coming to an additional rehearsal session the week before their juries (musical final exams.) In order to decide whether these extra sessions actually help students, he keeps track of who attends them and compares their jury scores to those of students who did not schedule extra sessions.
In this study, a difference was found between the scores of students who scheduled the extra sessions and those of students who didn't attend (higher average for students with extra sessions) . Does this study give evidence that attending the review session leads to higher grades on their juries?

A) Yes, because the study included both students who attended the review session and those who did not.
B) Yes, because the number of subjects in the study was large.
C) Yes, because the study relied on a common final exam for everyone.
D) No, because the students chose whether they attended the review session or not.
E) No, because the students in the study were not blinded.