Question

with a flexible exchange rate system what determines the exchange rate between two countries
A. the volume of gold
B. inflation and interest rates
C. the day-to-day changes in one currency
D. supply and demand

Answer 1

D.

I believe you mean a floating exchange rate, and if so, then it would be demand and supply which determines the rate of exchange.

Answer 2

supply and demand

exchange rate between countries depends on their amount of goods, and their need of goods