Which of these is not a cause of rightward shift of Oxygen - Hemoglobin dissociation curve?

Bohr effect is closely related to the fact that deoxygenated hemoglobin (deoxyhemoglobin) binds H+ more actively than does oxygenated hemoglobin (oxyhemoglobin). The pH of blood falls as its CO2 content increases, so that when the PCO2 rises, the curve shifts to the righ A rise in temperature or a fall in pH shifts the curve to the right HbO2 + 2,3-BPG - - Hb - 2,3-BPG + O2 In this equilibrium, an increase in the concentration of 2,3BPG shifts the reaction to the rightRef: Ganong's Review of Medical Physiology Twenty-Third Edition Page No:611