(R)-2-Bromo-1-chloropropane, $CH X_{3} - CH (Br) - CH X_{2} Cl$ , is treated separately with $CH X_{3} COOK$ in DMSO (giving A) and with $KCN$ in DMSO (giving B), each by pure $S_{N} 2$ at the C-Br bond. The labels of A (1-chloropropan-2-yl acetate) and B (3-chloro-2-methylpropanenitrile) are respectively

Answer: C

Both reactions invert the spatial arrangement. For the acetate,

O > CH X_{2} Cl > CH X_{3} > H

, the same order as the bromide (

Br > CH X_{2} Cl > CH X_{3} > H

), so the label flips to (S). For the nitrile,

CH X_{2} Cl

(Cl,H,H) outranks

CN

(N,N,N), so the first two priorities swap, which cancels the effect of inversion and the label remains (R).