${\mathrm{CH}}_2\mathrm{Cl}-{\mathrm{CH}}_2\mathrm{Cl}\stackrel{\text{ alc. KoH }}{⟶}A\stackrel{{\mathrm{NaNH}}_2}{⟶}B\frac{\text{ dil. }{\mathrm{H}}_2{\mathrm{SO}}_4}{{\mathrm{HgSO}}_4,{60}^0\mathrm{C}}\mathrm{C}\left(\text{ final }\right)$. Functional isomer of ‘C’ is

Ethylene dichloride upon reaction with alcoholic KOH and sodamide undergoes dehydrohalogenation gives acetylene which on hydrolysis with dilute sulphuric acid dilute mercuric sulphate forms vinyl alcohol which tautomerizes to acetaldehyde.