Solution

$$\textbf {Cross aldol condensation:}$$ Condensation between 2 different aldehydes or ketones, at least one of which contains ∝ – hydrogen is called aldol condensation. It is a useful synthetic tool only when one of the aldehyde or ketone does not contain $$∝ –$$ hydrogen. This is because if both the aldehydes or ketones contain $$∝ –$$ hydrogen it gives a mixture of four different products.
eg:- Condensation between formaldehyde and acetaldehyde in the presence of dil $$NaOH$$ (aq)