In a Simple Circular-Flow Diagram, How Are Total Income and Total

In a simple circular-flow diagram, how are total income and total expenditures in an economy interrelated?

A) They are equal because firms re-invest their revenue.
B) They are equal only if there is no saving.
C) They never equal because some people's income does not come from production activities.
D) They are equal because every transaction has a buyer and a seller.