Which of the following is an example of $S_N2$ reaction?

• A
  $CH_3Br + OH^{-} \to CH_3OH + Br^{-}$
• B
  $CH_3-CH(Br)-CH_3 + OH^{-} \to CH_3-CH(OH)-CH_3 + Br^{-}$
• C
  $CH_3CH_2OH \xrightarrow{-H_2O} CH_2=CH_2$
• D
  $CH_3-C(CH_3)(Br)-CH_3 + OH^{-} \to CH_3-C(CH_3)(OH)-CH_3 + Br^{-}$