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': "Ideal position for dentist to stand while taking radiographs \nSelect and explain. \nA. Behind head of patient, B. At an angle of 90-135 degrees and six feet away from patient, C. In 11'o clock position, D. At an angle of 180° and 9 feet away", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]