Indicate whether each of the following is counted in the United States gross domestic product for the year 2006. Explain your answer.
a. The value of used textbooks sold through an online auction in 2006.
b. Rent paid in 2006 by residents in an apartment building in 2000.
c. Commission earned in 2006 by a stockbroker.
d. The value of automobiles produced in 2006 entirely in South Korea by a firm fully owned by United States citizens.
