The secondary bromide shown (configuration not specified) can react by $\mathrm{S}{}_{\mathrm{N}}1$ or $\mathrm{S}{}_{\mathrm{N}}2$ depending on conditions. Which of the following changes would favour $\mathrm{S}{}_{\mathrm{N}}2$ over $\mathrm{S}{}_{\mathrm{N}}1$? Select all that apply.

The compound is 2-bromobutane. A high concentration of a good anionic nucleophile (azide) raises the bimolecular rate, and a polar aprotic solvent such as DMF leaves the anion unsolvated and more reactive. Ag+ pulls off bromide and promotes ionisation ($\mathrm{S}{}_{\mathrm{N}}1$), and water as solvent without added nucleophile favours solvolysis through the carbocation. Correct: the first two.