Product $(C)$ of the above reaction is

Which of the following is a solid trimer of formaldehyde?

The reaction that does not give benzoic acid as the major product is

Among the following,the order of reactivity towards nucleophilic addition is:

Which of the following substances will not give a brick-red precipitate with Fehling's solution?

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 $3$.