This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Randomisation is useful to eliminate:(1994) \nSelect and explain. \nA. Observer bias, B. Confounding factors, C. Patient bias, D. Sampling bias', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans: bRef: Park, 17th ed, p. 72 & 18th ed, p. 76', 'role': 'assistant'}]