Which member of the following pairs of nuclides would you expect to be most abundant in natural sources: (a) $_{20}^{40} \mathrm{Ca}$ or $_{20}^{42} \mathrm{Ca} ;$ (b) $_{15}^{31} \mathrm{P}$ or $_{15}^{32} \mathrm{P} ;$ (c) $_{30}^{63} \mathrm{Zn}$ or
64 $\mathrm{Zn} ?$ Explain your reasoning?