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) Their surface area to volume will decrease, making it easier for them to fit through arteries and veins
B) Provides more membrane for the electron transport chains where oxygen is consumed
C) Provides more sites for the membrane-bound hemoglobin molecules which transport oxygen
D) Saves energy by making more membrane relative to the volume of the cell
E) Maximize the relative surface area to exchange gasses