A researcher thinks that a person's mood has an effect on how helpful that person is likely to be. To test this, the researcher has participants come to the laboratory where they are first given a difficult test and then either heavily praised

or strongly criticized for their test performance. As it happens, the researcher has one of two research assistants, one who is a bit rude and the other more friendly, running each participant through the experimental procedure. After the praise or critical feedback, each participant is asked to help the research assistant in moving some heavy boxes into another room. The researcher observes how many boxes each participant moves in helping the research assistant. One possible confound for this experiment is ________.
A) the researcher's notes and observations
B) the participants' mood after the praise or criticism
C) whether or not participants showed up for the experiment
D) the attitude of the assistant
E) the number of boxes the assistant moved

Answer: D