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: Radiology
[{'content': 'Units of radiation exposure is: \nSelect and explain. \nA. Rad, B. Roentgen, C. Rem, D. Seivert', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]