This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Physics of Radiotherapy, Subject name: Radiology
[{'content': 'Unit of radiation exposure: \nSelect and explain. \nA. Rad, B. Gray, C. Sievert, D. Roentgen', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. (d) RoentgenRef: Essentials of radiological Science 2nd ed. 13* Unit of radiation exposure- Roentgen (SI= Coloumb/ kg)* Unit of absorbed radaition- RAD (SI= GRAY)* Unit of biologic effectiveness- Rem (SI= Sievert)', 'role': 'assistant'}]