The shape of demand curves firms face may change from one form of market structure to another. For instance, perfectly competitive oligopoly firms do not face the same demand curves due to various reasons.
There are many dairy farmers in the world and also many Starbucks coffeehouses.
Why does a Starbucks coffeehouse face a downward-sloping demand curve while a dairy farmer has a horizontal demand curve?
What other suppliers might face a downward-sloping demand curve?
What implications does this have for their advertising budget as compared to suppliers with horizontal demand curves?
