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 X-ray collimators commonly used in dentistry include\nthe following, except \nSelect and explain. \nA. Diaphragm collimator, B. Tubular collimator, C. Rectangular collimator, D. Square collimator', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]