An experimenter finds that students in a morning class perform better on a math test after being taught with a new teaching method compared with students in an afternoon class who are taught with a standard teaching method. He concludes that the new teaching method is superior to the old method. The main problem with this experimenter's conclusion is: a. He did not consider the time of day that students were being taught.
b. He did not find a random samplopf participants.
c. He did not consider the possible influence of participant bias.
d. He did not consider the importance of external validity