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': 'Half value layer refers to \nSelect and explain. \nA. The rate at which an X-ray photon transfer its energy to irradiated matter, B. The thickness of a substance required to reduce the number of X-ray photons by half, C. The time taken for X-ray photons to travel half the distance from the source to the object, D. The heel-effect seen when the anode is placed at an angle to the electron stream in the X-ray cathode tube', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]