What Is the Selective Advantage of a Mature Red Blood

What is the selective advantage of a mature red blood cell being flat rather than a sphere?

A) It provides more membrane for the electron transport chains where oxygen is consumed.
B) It maximizes the relative surface area available to exchange gasses.
C) The cell's surface area to volume will decrease, making it easier for the cell to fit through arteries and veins.
D) It provides more sites for the membrane-bound hemoglobin molecules which transport oxygen.
E) It saves energy by making more membrane relative to the volume of the cell.