Which of the following applies to "Bohr effect"?

Question

(1) Effect of $CO_2$ and $H^+$ on the oxygen affinity of Haemoglobin

(2) Partial pressure of $O_2$ in Pulmonary veins

(3) Exchange of chloride and bicarbonate ions between RBC and plasma

(4) Partial pressure of $ O_2$ in systemic arteries