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': 'Reducing the size of X-ray beam is achieved by \nSelect and explain. \nA. Filtration, B. Photo electric effect, C. Collimation, D. Bezold-brucke effect', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]