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': 'The dental x-ray beam is collimated to a circle of: \nSelect and explain. \nA. 7 cm in diameter, B. 5 cm in diameter, C. 6 cm in diameter, D. 9 cm in diameter', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Collimation is a process by which x-ray beam size is reduced and thereby decrease the patient exposure.\xa0\nIt is made up of metallic barrier usually Pb with an aperture in the middle to reduce the size of the x-ray beam.\nMost state regulations require that the x-ray beam used in intraoral radiography\xa0 be\xa0 collimated\xa0 so\xa0 that\xa0 the\xa0 field\xa0 of\xa0 radiation\xa0 at\xa0 the patient’s skin surface is no more than 7 cm (2.75 inches) in diameter.', 'role': 'assistant'}]