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': 'Which of the following is a naturally occurring radioactive substance in the body in small quantities ? \nSelect and explain. \nA. Radium 226, B. Bismuth 60, C. Iodine 131, D. Potassium 40', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]