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': 'Among the following, which one is not a major contributor to arid rains- \nSelect and explain. \nA. Carbon dioxide, B. Carbon monoxide, C. Nitric oxide, D. Sulphur dioxide', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Carbon monoxide", 'role': 'assistant'}]