Which compound has the highest reactivity towards $S_{N}2$ reaction?

Benzyl bromide on reaction with magnesium in the presence of dry ether gives $'X'$. $'X'$ on reaction with ethanol gives $'Y'$. Then the compound $Y$ is

$n-$Heptane $\xrightarrow[\Delta ]{Al_2O_3} P$ $\xrightarrow{[1 \ mole]}{NBS} Q$. Product $Q$ will be

The major product is:

$A$ $(C_{4}H_{8}Cl_{2})$ $\xrightarrow{\text{Hydrolysis at } 373 \text{ K}}$ $B$ $(C_{4}H_{8}O)$
$B$ reacts with hydroxylamine but does not give Tollen's test. Identify $A$ and $B$.

Match the $CH_{3}X$ compounds in Column-$(I)$ with their $C-X$ bond length in Column-$(II)$ and dipole moment in Column-$(III)$.

$(I)$ Compound	$(II)$ $C-X$ Bond Length $(pm)$	$(III)$ Dipole Moment $(D)$
$(A)$ $H_3C-F$	$(i)$ $214$	$(1)$ $1.636$
$(B)$ $H_3C-Cl$	$(ii)$ $193$	$(2)$ $1.830$
$(C)$ $H_3C-Br$	$(iii)$ $178$	$(3)$ $1.847$
$(D)$ $H_3C-I$	$(iv)$ $139$	$(4)$ $1.860$