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: Unknown
True regarding double blinding placebo controlled clinical trial: 
Select and explain. 
A. Some patients are not treated, B. The clinician does not know which treatment is given to the patient, C. 50% of the patients do not know what drug they receive, D. Everybody receives both the drugs
B: The clinician does not know which treatment is given to the patient
Answer: B: The clinician does not know which treatment is given to the patient 
Explanation: (Refer: K. Park’s Textbook of Preventive and Social medicine, 24th edition, pg no: 78)
Bias

Any systematic error in an epidemiological study occurring during data collection, compilation, analysis and interpretation
Types of bias