Answer:
Copper and Silvers are used by the US government to back the paper currency it printed.
The printing of Paper money in the United States started in 1690 and represented bills of credit.
In order to finance the Civil War In 1861, new currencies were introduced in the U.S..
Also In 1996, new bills were introduced to prevent counterfeiting of the Paper currency.
In conclusion, the United State has backed the Paper currency in all following way in the question except use of Gold and government's promise to pay.
Step-by-step explanation: