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
All are example for randomised control trials except - 
Select and explain. 
A. Natural experiments, B. Clinical trials, C. Risk factor trials, D. Preventive trials
A: Natural experiments
Answer: A: Natural experiments 
Explanation: Ans. is 'a' i.e., Natural experiments Types of randomized controlled trials Clinical trials 3. Risk factor trials 5. Trial of etiological agents Preventive trials 4. Cessation experiment 6. Evaluation of health services Types of nonrandomized trials. Uncontrolled trials Natural experiments Before and after comparison studies