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': 'A study has been done to establish the relationship between smoking and lung cancer. It was found that the association was more in people who exercise less and less in people exercise more. In this situation, exercise is a \nSelect and explain. \nA. Bias, B. Confounding, C. Effect modifier, D. Collinear factor', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. c. Effect modifier', 'role': 'assistant'}]