Identify reagent $B$ in the following reaction: $R - X \xrightarrow{B} \text{Nitroalkane}$

Which of the following pairs are correctly matched?
| | Reactants | Products |
|---|---|---|
| $I.$ | $RX + AgOH(aq)$ | $RH$ |
| $II.$ | $RX + AgCN(alc)$ | $RNC$ |
| $III.$ | $RX + KCN(alc)$ | $RNC$ |
| $IV.$ | $RX + Na(ether)$ | $R-R$ |

Arrange the following three chlorides in decreasing order of their reactivity towards $S_{N^1}$ reaction.

$A$ chloro compound $A$:
$(i)$ forms aldehydes on ozonolysis followed by hydrolysis.
$(ii)$ when vaporized completely,$1.53 \ g$ of $A$ gives $448 \ mL$ of vapour at $STP$.
The number of carbon atoms in a molecule of compound $A$ is ...... .

For the following reactions:
$CH_{3}CH_{2}CH_{2}Br + Z^{-} \xrightarrow[substitution]{k_s} CH_{3}CH_{2}CH_{2}Z + Br^{-}$
$CH_{3}CH_{2}CH_{2}Br + Z^{-} \xrightarrow[elimination]{k_e} CH_{3}CH=CH_{2} + HZ + Br^{-}$
where
$Z^{-} = CH_{3}CH_{2}O^{-} (A)$ or $(CH_{3})_{3}CO^{-} (B)$
$k_s$ and $k_e$ are,respectively,the rate constants for the substitution and elimination,and $\mu = \frac{k_s}{k_e}$,the correct option is:

Alkyl iodides on keeping become dark in color due to ...........