Which of the following haloalkanes is most reactive towards nucleophilic substitution reactions?

Observe the following reactions:
$1$. $C_6H_5-CH=CH_2 + HBr \rightarrow X$
$2$. $C_6H_5-C(CH_3)=CH_2 + HBr \rightarrow Y$
$3$. $C_6H_5-CH=CH_2 + HBr \xrightarrow{(C_6H_5COO)_2} Z$
The correct order of reactivity of $X, Y, Z$ towards $S_N1$ reaction is:

Identify the product $B$ in the following reaction sequence:
$1$-bromo-$4$-chlorobenzene $\xrightarrow[\text{Dry ether}]{Mg (1 \text{ equivalent})} (A)$ $\xrightarrow{HO-CH_2-CH_2-Cl} (B)$

Which of the following will most readily give the dehydrohalogenation product?

Assertion : $CHCl_3$ is stored in dark bottles.
Reason : $CHCl_3$ is oxidised in dark.

Identify the correct order of reactivity for the $E_2$ reaction with alcoholic $KOH$ for the following compounds:
$(i)$ $(CH_3)_3C-Cl$
(ii) $(CD_3)_3C-Cl$
(iii) $(CT_3)_3C-Cl$