Optically pure (S)-2-iodobutane is stirred with $\mathrm{NaI}$ in dry acetone. Iodide exchange occurs by SN2 only. The optical activity of the solution gradually falls to zero because

1. Aeach exchange inverts one molecule, so the S and R enantiomers finally become equal in amount
2. Ba planar carbocation forms in acetone
3. Ciodide attacks from the same side as the leaving group, giving partial retention
4. Dthe S enantiomer is converted into a meso compound