Terminal alkynes ($\mathrm{p}{K}_a\approx 25$) can be converted almost completely into sodium alkynides by which of the following?

A reaction goes forward when the acid is stronger than the conjugate acid of the base. $\mathrm{NH}{}_3$ (conjugate acid of $\mathrm{NH}{}_2{}^{-}$) has $\mathrm{p}{K}_a\approx 38$, much higher than 25, so $\mathrm{NaNH}{}_2$ deprotonates the alkyne completely. Water (15.7) and $\mathrm{H}{}_2\mathrm{CO}{}_3$ (6.4) are far stronger acids than the alkyne, so $\mathrm{OH}{}^{-}$ and $\mathrm{HCO}{}_3{}^{-}$ cannot do it.