Which of the following reagents can be used to distinguish between an aldehyde and a ketone?

Which one of the following is not correctly matched?

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 $3$.

What is meant by the following terms? Give an example of the reaction in each case.
$(i)$ Cyanohydrin
$(ii)$ Acetal
$(iii)$ Semicarbazone
$(iv)$ Aldol
$(v)$ Hemiacetal
$(vi)$ Oxime
$(vii)$ Ketal
$(viii)$ Imine
$(ix)$ $2,4-DNP$ derivative
$(x)$ Schiff's base

$2-$pentanone can be distinguished from $3-$pentanone by the reagent:

Which one is most reactive towards nucleophilic addition reaction?