A Professor of Medical-Surgical Nursing Researches the Effect of Special

A professor of medical-surgical nursing researches the effect of special tutoring sessions offered once weekly to students, in hopes of improving student performance on examinations. In the fall semester, student exam scores were significantly improved over those of the previous term, when tutoring was not offered. The performance of the spring term students does not show a significant improvement over students without tutoring. The professor's colleague identifies a confounding variable. Which variable is the confounding variable?

A) The same professor provided tutoring in both fall and spring semesters.
B) The fall semester students had a lower admission grade point average than the spring semester students.
C) The fall semester students were, on average, older than the spring students.
D) The fall tutoring sessions were conducted on Mondays after the medical-surgical class and the spring sessions were held on Fridays, after a different class.