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
Which one of the following therapeutic mode is commonly employed in intra-operative radiotherapy? 
Select and explain. 
A. Electron, B. Photon, C. X-ray, D. Gamma rays
A: Electron
Answer: A: Electron 
Explanation: An electron is a negatively charged subatomic particle.
It can be either free (not attached to any atom) or bound to the nucleus of an atom.
Electrons in atoms exist in spherical shells of various radii, representing energy levels.
The larger the spherical shell, the higher the energy contained in the electron.