What is the order of reactivity for the $S_{N}2$ reaction among the following alkyl halides?

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

The maximum number of $RBr$ isomers producing $2-$methylbutane by the given sequence of reactions is $...........$ $(Consider the structural isomers only)$

Identify the major product $Y$ in the given reaction sequence.

The product of the reaction is:

Which of the following rules applies in the reaction:
$CH_3CHBrCH_2CH_3 \xrightarrow{alc. KOH}$
$(i)$ $CH_3CH = CHCH_3$ (major product)
$(ii)$ $CH_2 = CHCH_2CH_3$ (minor product)