71.The concentration of oxygen is decreased at high altitude. Why would this make it harder for a person to get enough oxygen into their blood? (A) Hemoglobin cannot bind oxygen well at low concentrations；(B) The concentration gradient from the alveoli to the blood decreases； (C) People are working harder at high altitude and have higher demands for oxygen；(D) The concentration of oxygen in the blood is too low for diffusion into tissues.