Which of the following ions are necessary for assembly of microtubules?

(a) $N{a}^{+}$ and ${\mathrm{K}}^{+}$

(b) ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Cl}}^{-}$

(c) ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$

(d) ${\mathrm{Na}}^{+}$ and ${\mathrm{Ca}}^{2+}$