The reaction of phthalaldehyde with concentrated $NaOH$ produces:

$CH_3CHO$ $\xrightarrow{HCN} A$ $\xrightarrow{H_3O^+} B$. The product $B$ is

The end product $(C)$ in the following sequence of reactions is:
$HC \equiv CH$ $\xrightarrow[20\% \ H_2SO_4]{1\% \ HgSO_4} A$ $\xrightarrow[H_2O]{CH_3MgX} B$ $\xrightarrow{[O]} (C)$

The major product of the following reaction is:

What is an acetal? Give examples. $OR$ Give the reaction of ethanal with $(a)$ ethanol and $(b)$ ethylene glycol in the presence of dry $HCl$ gas.

Which of the following aldehydes is less reactive towards nucleophilic addition reaction?