Which of the following is NOT an argument for using PPP exchange rates when making comparisons of GDP across countries? Select one: a. Exchange rates can be either fixed or floating, so discovering the appropriate exchange to use when converting GDP into a common currency is extremely difficult. b. PPP exchange rates allow GDP for different countries to be converted into a common currency while taking into account differences in price levels between countries. c. PPP exchange rates reflect the rates at which currencies would have to be converted to equalize the price of a standard basket of goods and services across countries. d. Spot exchange rates tend to change rapidly and frequently, while PPP exchange rates tend to fluctuate less