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': 'Compared to round collimation, rectangular collimation ↓es exposure by: \nSelect and explain. \nA. 60%, B. 50%, C. 40%, D. 30%', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Compared to round collimation, rectangular collimation ↓es exposure by 60%', 'role': 'assistant'}]