What Is the Advantage of a Mature Red Blood Cell

What is the advantage of a mature red blood cell having a flattened shape rather than a spherical shape?

A) makes 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) maximizes the relative surface area to exchange gasses