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': 'Unit of radiation exposure – \nSelect and explain. \nA. Rad, B. Rem, C. Gray, D. Roentgen', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Roentgen is the old unit and coulombs/kg is the new (SI) unit of radiation exposure.', 'role': 'assistant'}]