The rate of the ${\mathrm{S}}_{\mathrm{N}}2$ reaction $\mathrm{CH}{}_3\mathrm{I}+\mathrm{N}{}_3{}^{-}\to \mathrm{CH}{}_3\mathrm{N}{}_3+\mathrm{I}{}^{-}$ is far greater in DMF than in methanol. The best explanation is that

1. ADMF has a much higher dielectric constant than methanol, which dissociates the ions
2. BDMF hydrogen bonds to the transition state and stabilises it
3. Cin methanol the azide ion is hydrogen bonded and stabilised, whereas in DMF it is weakly solvated and therefore more reactive
4. DDMF is a stronger acid than methanol and protonates the leaving group