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 adding up the total expenditures of households and subtracting savings
B) by adding up the 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 total income paid by firms