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 'heel effect' results \nSelect and explain. \nA. In low intensity X-ray beam on anode side of central ray, B. In high intensity beam of X-rays towards cathode, C. Both of them are true, D. None of the above are true", 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]