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': 'Rotating anode is mainly used to: \nSelect and explain. \nA. Focus photons, B. To avoid heel effect, C. To dissipate heat from focal spot, D. Increase the energy of photons', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]