B  
2,3-Bisphosphoglycerate occupies a single binding pocket in the central cavity between the β-chains of one hemoglobin tetramer (1 site/tetramer) and stabilizes the T (deoxy) conformation, thereby lowering hemoglobin’s affinity for oxygen and shifting the O2-dissociation curve to the right [Johnson 1998, PMID 9826627; Nadolny 1993, PMID 8357536].