Optically pure (S)-2-bromopropanoate ion is hydrolysed in aqueous NaOH. Select the correct statement(s).

1. AAt high $[\mathrm{OH}{}^{-}]$ the direct SN2 attack by external hydroxide competes and the proportion of (R)-lactate formed increases.
2. BAt low $[\mathrm{OH}{}^{-}]$ the main product is (S)-lactate, formed with net retention.
3. CRetention arises because hydroxide displaces bromide from the front side in a single SNi step.
4. DThe retention arises from two successive SN2 inversions: carboxylate displaces bromide to give an alpha-lactone, and the lactone is then opened by water or hydroxide.