Which of the Following Are a Pair of Isotopes?
A) ${ }_{18}^{40} \mathrm{Ar}$

Multiple Choice

Which of the following are a pair of isotopes?

A) ${ }_{18}^{40} \mathrm{Ar}$ and ${ }_{19}^{40} \mathrm{K}$
B) ${ }_{20}^{40} \mathrm{Ca}$ and ${ }_{19}^{40} \mathrm{K}$
C) ${ }_{18}^{40} \mathrm{Ar}$ and ${ }_{36}^{80} \mathrm{Kr}$
D) ${ }_{20}^{40} \mathrm{Ca}$ and ${ }_{19}^{39} \mathrm{~K}$
E) ${ }_{20}^{40} \mathrm{Ca}$ and ${ }_{20}^{42} \mathrm{Ca}$