The number of isomeric carboxylic acids and esters possible for the formula ${\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_2$ are respectively

${\mathrm{CH}}_3{\mathrm{COC}}_2{\mathrm{H}}_5\stackrel{\mathrm{NaOH}/{\mathrm{I}}_2}{⟶} \underset{\mathrm{yellow} \mathrm{ppt}.}{\mathrm{X}} \stackrel{\mathrm{Ag}}{⟶}\mathrm{Y}\left(\mathrm{g}\right)$.

In this sequence, the gaseous product $\mathrm{Y}$ is

$\mathrm{x}$ may be

In the above reaction, product $\mathrm{P}$ is

The above reaction is known as

The acid $\mathrm{D}$ obtained through the following sequence of reactions is:

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Br}\stackrel{\mathrm{Alc}. \mathrm{KOH}}{⟶}\mathrm{A}\underset{{\mathrm{CCl}}_4}{\overset{{\mathrm{Br}}_2}{\to }}\mathrm{B}\underset{\left(\mathrm{excess}\right)}{\overset{\mathrm{KCN}}{\to }}\mathrm{C}\stackrel{{\mathrm{H}}_3{\mathrm{O}}^{+}}{⟶}\mathrm{D}$