A large proportion of oxygen is left unused in the human blood even after its uptakes by the body tissues. This $O_2$ :

Question

( A ) helps in releasing more $O_2$ to the epithelial tissues
( B ) acts as a reserve during muscular exercise
( C ) is enough to keep oxyhaemoglobin saturation at 96%
( D ) raises the $pCO_2$ of blood to 75 mm of Hg