<prompt> Biconcave shape of RBCs is helpful because:Easily passage through capillariesRoulex formation.Increased surface area for a given diameter.None.  <response>: Guyton-419Normal red blood cells, are biconcave discs having a mean diameter of about 7.8 micrometres and a thickness of 2.5 micrometres at the thickest point and 1 micrometre or less in the centre. The shapes of red blood cells can change remarkably as the cells squeeze through capillaries. 