This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Bio-Chemistry), Subject name: Biochemistry
Substance with same atomic number but different mass number: 
Select and explain. 
A. Isotope, B. Isobar, C. Isomer, D. Molecule
A: Isotope
Answer: A: Isotope 
Explanation: Ans. a (Isotope). (Ref. Radioactive isotope in Medicine Biology by Quimby, 2nd ed., 26)TERMDEFINITIONE-g.IsotopesAtoms having same atomic numbers, different mass numbers (hence different neutron number)e.g., 17CI35, 17CI37IsobarsAtoms having same mass numbers, different atomic number (hence different neutron number)e.g., 28Ni64, 30Zn64IsotonesAtoms having same neutron number, different atomic mass numbere.g., 18A40IsomersAtoms having same atomic and mass numbers but different energy states in nucleus