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': 'Mean exposure of radiation from an intraoral periapical radiograph: \nSelect and explain. \nA. 300 mR, B. 90 mR, C. 0.03 R, D. 10 mR', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Mean exposure of radiation from an intraoral periapical radiograph\xa0is 300 mR.', 'role': 'assistant'}]