Atomic numbers: Eu 63, Tb 65, Gd 64, Cm 96, Am 95, Bk 97. Ground states: Eu $4f^{7}6s^2$, Tb $4f^{9}6s^2$, Cm $5f^{7}6d^{1}7s^2$, Am $5f^{7}7s^2$, Bk $5f^{9}7s^2$. In which pair do both ions have the same number of f electrons?

1. A$\mathrm{Tb}{}^{3+}$ and $\mathrm{Bk}{}^{4+}$
2. B$\mathrm{Eu}{}^{3+}$ and $\mathrm{Cm}{}^{3+}$
3. C$\mathrm{Eu}{}^{2+}$ and $\mathrm{Cm}{}^{3+}$
4. D$\mathrm{Gd}{}^{3+}$ and $\mathrm{Am}{}^{3+}$