How is glacial acetic acid obtained?

In the reaction $R - X$ $\xrightarrow[\text{alcoholic}]{KCN} A$ $\xrightarrow[\Delta]{H_2O/H^\oplus} B$,product $(B)$ is

What is the final product of the following reaction sequence?
$C_4H_8$ $\xrightarrow[\text{peroxide}]{\text{HBr}} X$ $\xrightarrow[\text{(ii) } CO_2]{\text{(i) Mg/dry ether, (iii) } H_3O^+} Y$

Hydrolysis of acetonitrile in acidic medium produces

Formic acid is obtained when

Identify product $C$ in the following conversion sequence: $m$-Hydroxybenzaldehyde $\xrightarrow[\text{Protection of } -OH \text{ group}]{C_6H_5CH_2Cl} A$ $\xrightarrow{[O]} B$ $\xrightarrow{\text{deprotection}} C$.