Reconsider the previous study about GPAs and pulling all-nighters. Suppose that the difference between these two averages (3.1 vs. 2.9) is shown to be statistically significant. Which of the following is a potential confounding variable and provides an alternative explanation for why the all-nighter group would have a significantly lower average GPA?
A. Students
B. Type of lifestyle choices made by student
C. Difficulty level of classes offered at St. Lawrence
D. Class sizes

Refer to a study published in the January 2008 issue of Behavioral Sleep Medicine that involved a survey of 120 students at St. Lawrence University, a small liberal arts college in upstate New York.