Debriefing a Subject at the End of an Experiment

Debriefing a subject at the end of an experiment:

A) is unethical if the subject already seems upset by the experimental procedure
B) is a valuable way of undoing some of the discomfort and deception that may have occurred during the experiment
C) has been shown to be relatively ineffective in undoing possible harmful effects to the subject
D) helps to keep the subject unaware of the true nature of the experiment