Answer:
single-product demand curve assumes constant money income such that a lower price causes a substitution of the now relatively cheaper product for those whose prices have not changed.
Step-by-step explanation:
When the aggregate demand curve i.e. downward sloping would be different to the demand curve for the single product i.e. also downward sloping is due to as the single product demand curve would assume that the income would be constant in such a way the less price would lead a substitution that the product is not expensive at all
So the above would be the reason