A Double-Blind Study of a Vaccine Is One in Which

A double-blind study of a vaccine is one in which:

A) the study group receives the vaccine and the control group receives a placebo.
B) neither observers nor subjects know the nature of the placebo.
C) neither observers nor subjects know which subject receives the vaccine and which receives a placebo.
D) neither the study group nor the control group knows the identity of the observers.
E) the control group does not know the identity of the study group.

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