What is the best explanation for the billions in Americancurrency being held abroad?

Select one:

a. U.S. multinational corporations are holding the money but notreporting it.

b. Drug dealers and others in the underground economy areholding the money.

c. Residents in foreign nations are using U.S. currency asstable currency to make transactions

d. There has been a proliferation of automatic teller machinesthroughout the world that allow people access to U.S. currency.