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': 'When the time of exposure is doubled: \nSelect and explain. \nA. The number of photons generated is doubled, B. The energy of photons generated is doubled, C. The velocity of photons is doubled, D. The mass of photons is doubled', 'role': 'user'}, {'content': 'Answer: A \nExplanation: EXPOSURE TIME (s):\nChanging the exposure time—typically measured in fractions of a second (s)—modifies the duration of the exposure and thus the number of photons generated. When the exposure time is doubled, the number of photons generated at all energies in the X-ray emission spectrum is doubled. The range of photon energies is unchanged.', 'role': 'assistant'}]