Professor Knudsen wanted to test the effect of a new method of second language instruction for young adults speaking only one language. She introduced the new method in all three of her spring semester German classes. At the end of the semester, she averaged the student grades. Since the overall class average was a "B," she concluded that the new method enhanced acquisition of the second language.

What’s wrong with this study? From the list of flaws below, select the major flaw in this study.
Select one:
a. no control group
b. biased or nonrepresentative sample
c. interpreting a correlation as indicating cause and effect
d. experimenter bias (experimenter’s expectations could influence the results)
e. participant bias (participants’ expectations could influence the results)