Blood consists of plasma and blood cells, including red blood cells, white blood cells and platelets. Red blood cells are rich in hemoglobin. Hemoglobin is a red iron protein, which combines with oxygen in places with high oxygen concentration and separates from oxygen in places with low oxygen concentration. Tibet is a plateau area, and the oxygen content in the air is less than that in the plain area. When people in plain areas arrive in plateau areas, their bodies will adjust accordingly, and the number of red blood cells and hemoglobin in their blood will increase.
So choose: D.