In an Economy Consisting of Only Households and Firms, How

In an economy consisting of only households and firms, how can GDP be computed?

A) by calculating the average total expenditures of households and subtracting savings
B) by calculating the average income paid by firms only in exchange for labour
C) by adding up either total income paid by firms or total expenditures of households, but not both
D) by adding up both the total expenditures of households and the total income paid by firms