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': 'Welfare officer is appointed in factories when no of workers are more than \nSelect and explain. \nA. 350, B. 500, C. 1000, D. 5000', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]