A Psychology Instructor Studying the Effects of the Order of Test

A psychology instructor studying the effects of the order of test items on test scores gave half the class a test in which the item order reflected the order in the textbook. The other half received the same questions in random order. What was the independent variable in this experiment?

A) The order of the items
B) Scores on the test
C) The difficulty of the items on the test
D) The method used to assign subjects to groups