The effect of the time of day a psychology class is taught on test scores is being studied in an experiment. The difficulty of test questions has been identified as an extraneous factor that could also affect test scores. How can the difficulty of test questions best be addressed to minimize its effect in the experiment? A. Direct control by requiring every class to take the same test B. Blocking into groups based on the teacher of the class C. Replication of the experiment at different times of the year D. Randomization of students in morning and afternoon classes