I believe the answer would be aiding in digestion and extracting carbon dioxide from blood.
Explanation: Bone marrow production stops when a child reaches maturity. White blood cells are produced from bone marrow, and O red cells are produced from red bone marrow, leaving the only answer left by process of elimination as B. Kidneys are also responsible for keeping ph balance, sodium and electrolyte balance, along with the balance of other vitamins and minerals in the blood.
