21. Which one of the following statements is correct assuming that exchange rates are quoted as units of foreign currency per dollar? a. The exchange rate moves opposite to the value of the dollar. b. The exchange rate rises when the U.S. inflation rate is higher than the foreign country's. c. The exchange rate declines as the foreign currency appreciates in value. d. The exchange rate falls as the dollar strengthens. e. The exchange rate is unaffected by differences in the inflation rates of the two countries