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': 'Phase 4 clinical trial is done as \nSelect and explain. \nA. To find safety and toxicity, B. To compare with existing drugs, C. Pre marketing surveillance, D. Post marketing surveillance', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The phases of clinical research are the steps in which scientists do experiments with a health intervention in an attempt to find enough evidence for a process that would be useful as a medical treatment.', 'role': 'assistant'}]