Identify all values of 
where r = 0. 
Definitions:
Sellers
Individuals or entities that offer goods or services for sale to consumers or other buyers.
Buyers
Individuals or entities that purchase goods or services for use or consumption.
Tax
A tax is a compulsory financial charge or some other type of levy imposed upon a taxpayer by a governmental organization in order to fund government spending and various public expenditures.
Price
The amount of money expected, required, or given in payment for something.
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