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': 'To restrict X-ray beam which of the following is done: \nSelect and explain. \nA. Collimation of beam, B. Increase Kvp, C. Decrease target object distance, D. Use a grid', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]