Which of the following reacts with alcohol in the presence of dry $HCl$ to form an acetal?

Two aliphatic aldehydes $P$ and $Q$ react in the presence of aqueous $K_2CO_3$ to give compound $R$,which upon treatment with $HCN$ provides compound $S$. On acidification and heating,$S$ gives the product shown below:
$1.$ The compounds $P$ and $Q$ respectively are:
$2.$ The compound $R$ is:
$3.$ The compound $S$ is:
Give the answer for questions $1$,$2$ and $3$.

In the reaction $CH_3CHO + HCN \to CH_3CH(OH)CN$,a chiral centre is produced. This product would be

List-$I$ contains various reaction sequences and List-$II$ contains the possible products. Match each entry in List-$I$ with the appropriate entry in List-$II$ and choose the correct option.

Identify the product $A$ in the following reaction:

What type of reaction is the formation of aldol from aldehyde?