Haemoglobin forms a compound with

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.

Haemoglobin has maximum affinity with

Each haemoglobin molecule has

In mammals,how much $CO_2$ is transported as bicarbonates of sodium and potassium in the blood?

Identify the correct sentence.