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': 'Which one of the following therapeutic mode is commonly employed in intra-operative radiotherapy? \nSelect and explain. \nA. Electron, B. Photon, C. X-ray, D. Gamma rays', 'role': 'user'}, {'content': 'Answer: A \nExplanation: An electron is a negatively charged subatomic particle.\nIt can be either free (not attached to any atom) or bound to the nucleus of an atom.\nElectrons in atoms exist in spherical shells of various radii, representing energy levels.\nThe larger the spherical shell, the higher the energy contained in the electron.', 'role': 'assistant'}]