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': 'Substance with same atomic number but different mass number – \nSelect and explain. \nA. Isotope, B. Isobar, C. Atom, D. Mineral', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Isotopes → Atoms of the same elements with same atomic number but different atomic mass (mass number).', 'role': 'assistant'}]