(R)-2-Bromo-1-methoxypropane, $CH X_{3} - CH (Br) - CH X_{2} - O - CH X_{3}$ , reacts with $NaCN$ in DMSO by a pure SN2 mechanism at the stereocentre. The product is $CH X_{3} - CH (CN) - CH X_{2} - O - CH X_{3}$ (3-methoxy-2-methylpropanenitrile). Its CIP label is

Answer: A

In the substrate the order is Br >

CH X_{2} OCH X_{3}

CH X_{3}

> H. In the product,

CH X_{2} OCH X_{3}

(O,H,H) outranks

CN

(N,N,N), so the order is

CH X_{2} OCH X_{3}

CN

CH X_{3}

> H. The entering group takes second rank where Br was first, which reverses the effect of geometric inversion, so the label remains R.