Select the correct statement about oxygen transport in blood

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