In the transport of ${\mathrm{CO}}_2$from the tissues to the lungs, which of the following occurs in venous blood?

Binding of ${\mathrm{HCO}}_3$^– to hemoglobin      

Conversion of ${\mathrm{CO}}_2$and ${\mathrm{H}}_2\mathrm{O}\text{ to }{\mathrm{H}}^{+}$and ${\mathrm{HCO}}_3$^– in RBC

Shifting of ${\mathrm{HCO}}_3$^– into the RBCs from plasma in exchange for $C{l}^{– }$

Buffering of ${\mathrm{H}}^{+}$ by oxyhemoglobin