Give the relation between the following pairs of species:
$(i)$ $_{20}^{40}Ca$ and $_{19}^{40}K$
$(ii)$ $_{17}^{35}Cl$ and $_{17}^{37}Cl$
$(iii)$ $_{18}^{40}Ar$,$_{20}^{40}Ca$ and $_{19}^{40}K$
$(iv)$ $_{38}^{87}Sr$ and $_{38}^{90}Sr$

(N/A) $(i)$ Isobars: Species with the same mass number $(A=40)$ but different atomic numbers.
$(ii)$ Isotopes: Species with the same atomic number $(Z=17)$ but different mass numbers $(A=35, 37)$.
$(iii)$ Isobars: All three species have the same mass number $(A=40)$ but different atomic numbers.
$(iv)$ Isotopes: Species with the same atomic number $(Z=38)$ but different mass numbers $(A=87, 90)$.