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
Substance with same atomic number but different mass number – 
Select and explain. 
A. Isotope, B. Isobar, C. Atom, D. Mineral
A: Isotope
Answer: A: Isotope 
Explanation: Isotopes → Atoms of the same elements with same atomic number but different atomic mass (mass number).