An organic compound $A$ is oxidized using acidified $K_2Cr_2O_7$ solution to form compound $B$. Compound $B$ gives the $DNP$ test but not the silver mirror test. Then,compound $A$ is:

In the following reactions,$P, Q, R$,and $S$ are the major products.
The correct statement$(s)$ about $P, Q, R$,and $S$ is(are):
$(A)$ Both $P$ and $Q$ have asymmetric carbon$(s)$.
$(B)$ Both $Q$ and $R$ have asymmetric carbon$(s)$.
$(C)$ Both $P$ and $R$ have asymmetric carbon$(s)$.
$(D)$ $P$ has asymmetric carbon$(s)$,$S$ does not have any asymmetric carbon.

Consider the given reaction,the product $A$ is :

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 $3$.

Explain the mechanism of nucleophilic addition reactions of aldehyde and ketone compounds.

The major product of the following reaction is