Answer: PURCHASING POWER PARITY.
Explanation: Purchasing power is defined as the amount of goods and services that can be bought with a unit of currency.
PURCHASING POWER PARITY refers to a theory of long-term equilibrium exchange rates based on relative price levels of two countries.
Exchange rates is determined by a whole number of factors some of which include; Differentials in Inflation; Differentials in Interest Rates; Current Account Deficits; Public Debt;etc.