In a $S_N2$ substitution reaction of the type $R-Br + Cl^- \xrightarrow{DMF} R-Cl + Br^-$,which one of the following has the highest relative rate?

The reaction $RCl + NaI \xrightarrow{\text{Acetone}} R-I + NaCl$ is known as:

Most reactive compound toward $S_N1$ is

For the following compounds,the correct statement$(s)$ with respect to nucleophilic substitution reactions is(are):
$I$: Benzyl bromide $(C_6H_5CH_2Br)$
$II$: Cyclohexylmethyl bromide $(C_6H_{11}CH_2Br)$
$III$: tert-Butyl bromide $((CH_3)_3CBr)$
$IV$: $1-$Phenylethyl bromide $(C_6H_5CH(CH_3)Br)$
$A$. $I$ and $III$ follow $S_{N}1$ mechanism
$B$. $I$ and $II$ follow $S_{N}2$ mechanism
$C$. Compound $IV$ undergoes inversion of configuration
$D$. The order of reactivity for $I$,$III$ and $IV$ is: $IV > I > III$

Which reaction is faster $(i)$ or $(ii)$ and what is the mechanism?
$(i) \ CH_3CH_2CH_2Br + CH_3ONa \xrightarrow{DMF} CH_3CH_2CH_2OCH_3 + Br^-$
$(ii) \ CH_3CH_2CH_2Br + CH_3SNa \xrightarrow{DMF} CH_3CH_2CH_2SCH_3 + Br^-$

Which of the following compounds is optically inactive?