ProfMartin Was Interested in Which of Two Popular Statistics Textbooks

Multiple Choice

Prof.Martin was interested in which of two popular statistics textbooks (Statistics: It Will Change Your Life and Statistics: Bigger, Better, Stronger) was better for students.Prof.Martin compared the two texts by assigning one text to a section of statistics taught by Prof.Miller from 10 to 11 a.m.on Monday, Wednesday, and Friday, and the other text to a section of statistics taught by Prof.Mervin from 7 to 10 p.m.on Wednesday evenings.At the end of the term, all students took the same comprehensive test.Students to whom Statistics: Bigger, Better, Stronger was assigned performed better on the test than did students to whom Statistics: It Will Change Your Life was assigned.Therefore, Prof.Martin concluded that the former textbook was the better one.Which of the following is NOT a potential confounding variable in this study?

A) number of classes per week
B) professors teaching course
C) comprehensive test scores
D) time of day course is taught