The oxygen - hemoglobin dissociation curve is adaptive because it:
a. shifts to the right at the lung capillaries and to the left in the tissue capillaries
b. only shifts to the left where there is more carbon dioxide and less oxygen
c. shifts in response to lower levels of 2, 3 BPG at the systemic capillaries
d. shifts to the left in the pulmonary capillaries and to the right in the systemic capillaries
e. doesn’t shift regardless of the carbon dioxide and oxygen partial pressures