A large proportion of oxygen remain unused in the human blood even after its uptake by the body tissues. This `O_(2)`
A. acts as a reserve during muscular exercise
B. raises the `PCO_(2)` of blood to `75 mm` of `Hg`
C. is enough to keeo oxyhaemoglobin saturation at `96%`
D. helps in releasing more `O_(2)` to the epithelial tissues.