What causes the dissociation of oxyhemoglobin?

Haemoglobin is a red-colored,iron-containing pigment present in $RBCs$. $O_2$ binds with it in a reversible manner to form..........

During $CO_2$ transport,the movement of chloride ions across the membrane of $RBCs$ from plasma is called . . . . . . .

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.

In which form is $CO_2$ mostly carried by blood?

Haemoglobin is