The following statements refer to the transportation of gases. Read them carefully and choose the correct option containing the incorrect statements.
$(i)$ After leaving the lungs, each litre of blood contains about $20 \ ml$ of oxygen.
$(ii)$ When $PCO_2$ rises, the affinity of haemoglobin for oxygen is increased.
$(iii)$ Each litre of arterial blood contains about $4 \ ml$ of $CO_2$.
$(iv)$ The role of oxygen in the regulation of respiratory rhythm is quite significant.

• A
  $(i)$ and $(ii)$
• B
  $(ii), (iii)$ and $(iv)$
• C
  $(i), (iii)$ and $(iv)$
• D
  $(i), (ii), (iii)$ and $(iv)$