Its stabilising. Since the selection pressure is against the extremes which are the normal blood and the sickle cell anaemia.
Answer:
When red blood cells are placed inside pure water they normally swell up and eventually explode. Red blood cells in pure water swell up because the pure water enter in the red blood cells by mean of osmosis. The water molecules are able to enter the red blood cells because the salt concentration inside the red blood cells is more than that of the pure water.
As water continue to move inside the cells, the cell swell up and eventually burst because the internal pressure inside the cell is greater than the external pressure, therefore the cell membrane become ruptured and eventually burst.
The answer would be letter a
