Toss Up - Biology - Multiple Choice

Which of the following would most hematologists agree is the MOST likely reason mammalian erythrocytes contain no nuclei or mitochondria:
W) to prevent oxidation of hemoglobin
X) to allow for more hemoglobin molecules per red blood cell
Y) to limit the lifespan of the red cell to 120 days
Z) to allow it to retain its bi-concave shape