This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Contrast Agents, Subject name: Radiology
[{'content': 'Which of the following is a radioactive substance? \nSelect and explain. \nA. Bromide, B. P32, C. C13, D. I127', 'role': 'user'}, {'content': 'Answer: B \nExplanation: P-32 is a radio-active substance. It is a beta emitter with a half-life of 14 days. It is used for systemic radiotherapy of Polycythemia Vera. I-127 is a normal iodine and it is not a radio-active substance', 'role': 'assistant'}]