Which of the following is an example of a capital input?

A) Buildings and machines used in the production process
B) Stocks and bonds
C) An hour of a worker's time
D) The money households use to purchase firms' output

Capital inputs refer to the physical assets used in the production process, such as buildings and machines. Stocks and bonds are financial assets, an hour of a worker's time is a labor input, and the money used by households to purchase output is a financial transaction, not a capital input.