Oxygen Levels Are Lower at Higher Altitudes

Oxygen levels are lower at higher altitudes. Why would you expect to see increased numbers of reticulocytes in the blood at high altitudes?

A) Erythrocyte production is being stimulated to increase the oxygen-carrying capacity of the blood.
B) More reticulocytes would increase the capacity of blood to fight infections, which are common at high altitude.
C) Monocyte production is being stimulated in order to deal with the low oxygen levels.
D) Reticulocytes become megakaryocytes, which produce more platelets for clotting at high altitudes.