An experimenter finds that students in a morning class perform better on a math test after being ...

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 find a random sample of participants.
  b. He did not consider the importance of external validity.
  c. He did not consider the possible influence of participant bias.
  d. He did not consider the time of day that students were being taught.