' $x$ ' is the product which is obtained from propanenitrile and stannous chloride in the presence of hydrochloric acid followed by hydrolysis. ' $y$ ' is the product which is obtained from the but-2-ene by the ozonolysis followed by hydrolysis. From the following, which product is not obtained when one mole of ' $x$ ' and one mole of ' $y$ ' react with each other in the presence of alkali followed by heating?

Product $x$ is obtained by the Stephen reduction of propanenitrile ($CH_3CH_2CN$).
$CH_3CH_2CN \xrightarrow{SnCl_2/HCl} CH_3CH_2CH=NH \xrightarrow{H_2O} CH_3CH_2CHO$ (Propanal).
Product $y$ is obtained by the reductive ozonolysis of but-2-ene ($CH_3CH=CHCH_3$).
$CH_3CH=CHCH_3 \xrightarrow{O_3, H_2O/Zn} 2CH_3CHO$ (Ethanal).
When propanal ($x$) and ethanal ($y$) react in the presence of alkali followed by heating, they undergo cross-aldol condensation.
Possible products are:
1. Self-aldol of propanal: $CH_3CH_2CHO + CH_3CH_2CHO \rightarrow CH_3CH_2CH=C(CH_3)CHO$ (2-Methylpent-2-enal).
2. Self-aldol of ethanal: $CH_3CHO + CH_3CHO \rightarrow CH_3CH=CHCHO$ (But-2-enal).
3. Cross-aldol (Ethanal as nucleophile): $CH_3CH_2CHO + CH_3CHO \rightarrow CH_3CH_2CH=CHCHO$ (Pent-2-enal).
4. Cross-aldol (Propanal as nucleophile): $CH_3CHO + CH_3CH_2CHO \rightarrow CH_3CH=C(CH_3)CHO$ (2-Methylbut-2-enal).
Comparing with the options, 3-Methylbut-2-enal is not formed in any of these reactions.