Select the events that occur in the erythrocytes in the systemic capillaries. 

A.  Dissociation of ${\mathrm{H}}_2{\mathrm{CO}}_3$ into ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$.

B.  Dissociation of ${\mathrm{H}}_2{\mathrm{CO}}_3$ into ${\mathrm{H}}^{+}$ and ${\mathrm{HCO}}_3^{-}$.

C.  Formation of ${\mathrm{H}}_2{\mathrm{CO}}_3$ from ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$.

D.  Formation of ${\mathrm{H}}_2{\mathrm{CO}}_3$ from ${\mathrm{H}}^{+}$ and ${\mathrm{HCO}}_3^{-}$.

At the tissue, carbonic acid facilitates the formation of ${\mathrm{H}}_2{\mathrm{CO}}_3$ from ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$. ${\mathrm{H}}_2{\mathrm{CO}}_3$ dissociates into ${\mathrm{H}}^{+}$ and ${\mathrm{HCO}}_3^{-}$. About 70% of ${\mathrm{CO}}_2$ is transported from the tissues to the lungs as ${\mathrm{HCO}}_3^{-}$. At the lungs, oxygen binds to haemoglobin, forming ${\mathrm{HbO}}_2$, which is more acidic and unloads ${\mathrm{H}}^{+}$. This ${\mathrm{H}}^{+}$ binds to ${\mathrm{HCO}}_3^{-}$ to form ${\mathrm{H}}_2{\mathrm{CO}}_3$, which dissociates ${\mathrm{CO}}_2$ and ${\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{CO}}_2$ is exhaled.