Complete each of the following sentences:

a. A product that serves both as money and as a commodity is ________________.

b. Some coins and all paper money circulating in the United States have face values that exceed the value of the materials from which they are made. Therefore, they are forms of _________ __________.

c. If the government declares that creditors must accept a form of money as payment for debts, the money becomes ________________________.

d. A common unit for measuring the value of every good or service in the economy is known as a(n) _____________________________________.