This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
The purpose of double blinding in clinical trials is to: 
Select and explain. 
A. Achieve comparability between study and control groups, B. Avoid observer bias, C. Avoid subject bias, D. Avoid observer and subject bias
D: Avoid observer and subject bias
Answer: D: Avoid observer and subject bias 
Explanation: Single blind: Only patients or only investigators are ignorant of assigned treatment.
Double blind: Patients & investigators are ignorant of assigned treatment.
Triple blind: Patients, investigators & data evaluators are ignorant of assigned treatment.