DrPine Is Testing a New Drug and Has a Placebo-Control

Multiple Choice

Dr.Pine is testing a new drug and has a placebo-control group.When he provides the drugs to patients and when he interviews them about side effects,he never knows whether the patient got the drug or the placebo.What is the most likely reason that Dr.Pine chose this type of research design

A)  It reduces the impact of experimenter bias. 
B)  It prevents the halo effect. 
C)  It minimizes the possibility of self-report bias. 
D)  It ensures that the sample is not biased.