(C) $1$. Isobars are atoms of different chemical elements that have the same mass number but different atomic numbers.$2$. In the pair $^{40}K_{19}$ a

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$^{40}K_{19}$ and $^{40}Ca_{20}$

(C) $1$. Isobars are atoms of different chemical elements that have the same mass number but different atomic numbers.$2$. In the pair $^{40}K_{19}$ and $^{40}Ca_{20}$, both atoms have a mass number of $A = 40$ but different atomic numbers ($Z = 19$ for Potassium and $Z = 20$ for Calcium).$3$. Therefore, they are isobars.