Correct Option: A

Red Blood cells contain haemoglobin which is what the oxygen binds with to form oxyhaemoglobin which is then transported to the different cells around the body. Oxygen bonds with the haemoglobin when it is at high partial pressure and then is released when there is a lower partial pressure of oxygen. At high altitudes there is lower atmospheric pressure of oxygen. This means that the current number of red blood cells in the body cannot meet the cells demands for oxygen. Due to the lower partial pressure of oxygen a process called polycythemia occurs, which is an increase in the bodies red blood cell count. The body increases its red blood cell count because this means there is more haemoglobin available to bond with oxygen molecules meaning more oxygen can be transported to the cells in the body, therefore helping to meet the oxygen demands of the body even with less oxygen in the air.