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': 'The source of endogenous radiation is – \nSelect and explain. \nA. Radon, B. Potassium, C. Thorium, D. Uranium', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]