This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Randomized controlled trials are \nSelect and explain. \nA. Clinical trials, B. Preventive trials, C. Before and after comparison studies, D. Evaluation of Health services', 'role': 'user'}, {'content': "Answer: A \nExplanation: Experimental studies intervention studies include: Randomized control trials Field trials Community trials Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 70", 'role': 'assistant'}]