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
[{'content': 'Concept of Web of causation was proposed by: \nSelect and explain. \nA. McMohan and Pugh, B. Louis Pasteur, C. John Snow, D. John Snow', 'role': 'user'}, {'content': 'Answer: A \nExplanation: McMohan and Pugh', 'role': 'assistant'}]