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': 'Randomisation is useful to eliminate ? \nSelect and explain. \nA. Observer bias, B. Confounding factors, C. Patient bias, D. Sampling bias', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Confounding", 'role': 'assistant'}]