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': 'In a study done to establish smoking as a risk factor for a disease, 30 out of 50 smokers developed the disease while 10 out of 50 non-smokers developed the disease. Odds ratio is? \nSelect and explain. \nA. 6, B. 3, C. 5, D. 10', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]