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: Social & Preventive Medicine
Randomization means - 
Select and explain. 
A. Mixing of cases and controls, B. Selecting cases, C. Selecting controls, D. Vaccine and Erythromycin
A: Mixing of cases and controls
Answer: A: Mixing of cases and controls 
Explanation: None