ProfMartin Was Interested in Which of Two Popular Statistics Textbooks

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.What was the independent variable in this study?

A) statistics textbooks
B) professors
C) comprehensive test
D) students