When acetaldehyde reacts with $PCl_{5}$,the resulting compound is

Compound $(A)$ with molecular formula $C_{11}H_8O$ does not undergo self-aldol condensation. Upon ozonolysis,it yields $Ph-CHO$ and $2B$,where $B$ on oxidation with $Ag^{+}$ gives oxalic acid. Identify compound $(A)$.

Compound $X$ undergoes the following reaction sequence. What is the structure of compound $X$?

Primary aldehyde on oxidation gives

Assertion $(A)$: Ammonia and its derivatives of the form $H_2N-Z$ undergo condensation reaction with carbonyl compounds (aldehydes and ketones).
Reason $(R)$: This reaction is an example of an irreversible reaction.
The correct option among the following is:

Which of the following is most reactive towards nucleophilic addition reaction?