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 of the following is/are the greatest contributor/contributors to global warming as a consequence of human activities and life style ? \nSelect and explain. \nA. Carbon dioxide, B. Chlorofluorocarbons, C. Methane, D. Ozone', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]