Anna,Barry,and Clara Grew Up Together in Michigan

Anna,Barry,and Clara grew up together in Michigan.After college,they each went overseas to countries that had different currencies.They had a group chat in which they were comparing costs of living.So that they could understand one another,they each converted their costs into U.S.dollars.What purpose did the U.S.dollar serve in their conversation?

A) Medium of exchange
B) Store of value
C) Unit of account
D) Loanable fund