Which of the Following Statements Describes a Double-Blind Experiment

Which of the following statements describes a double-blind experiment?

A) The experimental and control groups take turns getting each treatment.
B) Neither subjects nor researchers know which subjects are in the control group and which are in the experimental group.
C) Neither group of subjects knows whether they are in the control or experimental group, but the researchers do know.
D) Both subject groups know whether they are in the control or experimental group, but the researchers do not know.
E) Neither the subjects nor the persons having contact with the subjects know the true purpose of the experiment.