Professor Singh Conducts an Experiment to Test Whether or Not

Professor Singh conducts an experiment to test whether or not the Holmes Approach to Studying Psychology (HASP) is superior to the Watson Approach to Studying Psychology (WASP) .He convinces Dr.Ormrod to use HASP in her 9:00 am class and WASP in her 10:00 am class.Students exposed to the HASP method do better on the final exam than students exposed to the WASP method.Prof.Singh concludes that HASP is better than WASP.What is definitely wrong with Singh's experiment?

A) The final exam is not a good measure of what students have learned.
B) Students in the 9:00 am class are probably more motivated.
C) Students in the 9:00 am class are probably smarter.
D) There are several possible explanations for his results.