Classify the following reactions into one of the reaction types studied in this unit:
$(a)$ $CH_3CH_2Br + HS^{-} \rightarrow CH_3CH_2SH + Br^{-}$
$(b)$ $(CH_3)_2C=CH_2 + HCl \rightarrow (CH_3)_2ClC-CH_3$
$(c)$ $CH_3CH_2Br + HO^{-} \rightarrow CH_2=CH_2 + H_2O + Br^{-}$
$(d)$ $(CH_3)_3C-CH_2OH + HBr \rightarrow (CH_3)_2CBr-CH_2CH_3 + H_2O$

(A) This is a substitution reaction where the $-Br$ group is replaced by the $-SH$ group.
$(b)$ This is an addition reaction where $HCl$ adds across the double bond of isobutylene.
$(c)$ This is an elimination reaction (dehydrohalogenation) where $H$ and $Br$ are removed to form an alkene.
$(d)$ This is a substitution reaction accompanied by a carbocation rearrangement.