The oxygen--hemoglobin dissociation curve is sigmoid because
Binding of one oxygen molecule increases the affinity of binding other 02 molecules
Binding of one oxygen molecule increases the affinity of binding other 02 molecule
Binding of one oxygen molecule decrease the affinity of binding other 02 molecules
Oxygen affinity of Hemoglobin decreases when the pH of blood falls
Binding of oxygen to Hemoglobin reduces the affinity of Hb for CO2