Nitrobenzene and ${\mathrm{CH}}_3\mathrm{Cl}$ in presence of anhydrous ${\mathrm{AlCl}}_3$ gives:

Marsh gas mainly contains.

The correct order of electron density in the aromatic ring of the following compounds is :

       (I)                        (II)                          (III)                    (IV)

$\mathrm{R}-\mathrm{Cl}+\mathrm{AgCN}⟶\mathrm{A}\stackrel{\mathrm{Reduction}}{\to \mathrm{B}}$

$\mathrm{A}$ and $\mathrm{B}$ respectively are: