If a U.S. dollar purchases 4 Argentinean pesos, and a gallon of milk costs $3 in the U.S. and 6

If a U.S. dollar purchases 4 Argentinean pesos, and a gallon of milk costs $3 in the U.S. and 6 pesos in Argentina what is the real exchange rate?
a. 1 gallons of Argentinean milk/2 gallon of U.S. milk.
b. 2 gallons of Argentinean milk/1 gallon of U.S. milk.
c. 3 gallons of Argentinean milk/2 gallon of U.S. milk.
d. 2 gallons of Argentinean milk/3 gallon of U.S. milk.