Which of the Following Best Describes How One Would Determine

Which of the following best describes how one would determine a currency cross rate?

A) Calculating a currency cross rate involves determining the exchange rate for two currencies by using a widely held commodity index as a base.
B) Calculating a currency cross rate involves determining the exchange rate for two currencies by using a third currency as a base.
C) Calculating a currency cross rate involves determining the exchange rate for two currencies by using two other currencies as a base.
D) Calculating a currency cross rate involves determining the exchange rate for a basket of currencies by using a third currency as a base.