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 defines isotopes: September 2011 \nSelect and explain. \nA. Same atomic number and same mass number, B. Different atomic number and same mass number, C. Different atomic number and different mass number, D. Same atomic number and different mass number', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. Same atomic number and different mass number', 'role': 'assistant'}]