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': 'Traditional unit of radiation exposure is: \nSelect and explain. \nA. Air kerma., B. Roentgen., C. Coulomb/kg., D. Gray.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]