When 2-phenylacetaldehyde is added dropwise to benzaldehyde in the presence of sodium hydroxide they undergo a crossed aldol reaction to form 3-hydroxy-2,3-diphenylpropanal?

Carbonyl Compounds containing alpha proton readily donates alpha acidic proton when treated with strong Base. In given statement 2-phenylacetaldehyde is converted into Enolate and Enolates acts as a nucleophile and attacks carbonyl group of Benzaldehyde. The mechanism of given reaction is as follow, the movement of electrons is shown by RED arrows,