This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Radiotherapy, Subject name: Radiology
[{'content': 'Clinically useful radiation from cobalt 60 is \nSelect and explain. \nA. Gamma radiation, B. Beta radiation, C. Alpha paicles, D. Photons', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Cobalt units produce stable, dichromatic gamma beams of 1.17 and 1.33 MeV, resulting in an average beam energy of 1.25 MeV. The cobalt-60 isotope has a half-life of 5.3 years Reference: Eric J Hall Textbook of Radiobiology for Radiologist; 5th edition', 'role': 'assistant'}]