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[{'content': 'Which of the following elements is obsolete in radiotherapy \nSelect and explain. \nA. Radium 226, B. Cobalt 60, C. Iridium 192, D. Cesium 137', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Radium needles that were once implanted into tumours as a cancer treatment are now obsolete and constitute a\xa0radioactive waste problem, as their half-life is approximately 1600 years.\n\n\xa0 \xa0 \xa0 \xa0 \xa0 \xa0"Radioisotopes may be naturally occurring, such as radium - 226 (226 Ra)\' which is now seldom used in radiotherapy".', 'role': 'assistant'}]