Professor Hoffman Was Interested in the Effects of Vitamin B12

Professor Hoffman was interested in the effects of vitamin B12 deprivation on learning. One group is given vitamin B12 in a box labelled "Vitamin X." The comparison group is given a sugar pill in a plain white box. He finds that the vitamin group learns lists of words faster and concludes that B12 affects learning. What's wrong with this experiment?

A) It is confounded because of a possible experimenter bias effect.
B) It is confounded because there was no random assignment.
C) It is confounded because the study cannot be replicated.
D) There were demand characteristics present in the study.
E) There is nothing wrong with this experiment.