Identify the pairs which are not of isotopes?

(a) X 612 and Y 613 have same atomic number but different mass number.(b) X 1735 and Y 1737 have same atomic number but different mass number.Both these pairs are isotopes to each other.c) X 614 and Y 714 have different atomic number but same mass number.(d) X 48 and Y 58 have different atomic number but same mass number.Both these pairs are isobars to each other.