Which of the following reactions proceed predominantly by the ${\mathrm{S}}_{\mathrm{N}}2$ pathway?

A primary or methyl halide with a good anionic nucleophile in a polar aprotic solvent reacts by ${\mathrm{S}}_{\mathrm{N}}2$ (A and C). A tertiary halide in a polar protic ionising solvent (B, D) reacts by ${\mathrm{S}}_{\mathrm{N}}1$ via the tertiary carbocation, since backside attack is blocked.