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.