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Factors that affects the oxygen hemoglobin binding by increasing it's affinity are :

(i) Increased partial pressure of oxygen

(ii) decreses partial pressure of carbon dioxide.

(iii) High hydrogen ion concentration leading to low pH 

(iv) Low temperature

answer is option a bcuz for binding of O2 with hb it require following condition: 1.low temperature 2. low PCO2 3.high PO2 4. low ph

None for binding of o2 high o2 low co2 low temp. Low H+ Low H+ mean ph high