Why is system of currency exchange necessary for international trade?

a. nearly all world currencies are worthless on the world market.
b. those buying goods on the world market only accepted in gold and silver.
c. most goods bought on the international market must be bought and paid for using united states dollars only.
d. there must be a way to pay for goods purchased from countries with different types of currencies due to nationalism.
you for any answer that you provide! : d

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