A scientist claims that he has protected animal test subjects from a newly mutated virus using an experimental vaccine. The experimental vaccine was injected into five animal test subjects. The scientist also injected a placebo vaccine into five other animal test subjects. One month later, all of the test subjects were exposed to the mutated virus. The subjects that received the vaccination showed no sign of the disease and blood tests confirmed the presence of antibodies against the disease, but the subjects given the placebo developed symptoms of this condition. Why did one group have a placebo injected before being exposed to the virus? Choose 1 Choose 1 (Choice A) A They were part of the control group (Choice B) B They would be exposed to a weakened form of the virus (Choice C) C They would be protected from the disease (Choice D) D They were part of the experimental group