Select the correct statement about oxygen transport in blood: A. During conditions of acidosis, hemoglobin is able to carry oxygen more efficiently. B. During normal activity, a molecule of hemoglobin returning to the lungs carries one molecule of O2. C. A 50% oxygen saturation level of blood returning to the lungs might indicate an activity level higher than normal. D. Increased BPG levels in the red blood cells enhance oxygen-carrying capacity.