During gas exchange in the tissues
a. oxygen concentrations are much lower in the tissues so hemoglobin gives up oxygen
b. there is a high carbon dioxide concentration that causes hemoglobin to change shape
c. some carbon dioxide binds to the hemoglobin molecule
d. a and b
e. a, b and c

E. a, b and c

Explanation:

Gas exchange near tissues: Near the tissues the partial pressure of oxygen is less so oxyhemoglobin in the blood immediately dissociates to give away oxygen to the tissues.

Near the tissues the metabolized product carbon dioxide is more i.e. the partial pressure of carbon dioxide is more near tissues. A rise in partial pressure of CO2  of fall in pH, decreases oxygen affinity of hemoglobin, this is referred as Bohr's effect. Hemoglobin undergoes conformational changes that facilitates the release of oxygen.

Carbon dioxide must be transported from tissue to lungs for exhalation process. For this some part of CO2 (20-25%) binds with hemoglobin and gets transported as carbaminohemoglobin (HbCO2). Around 70% of CO2 is transported in the form of bicarbonate ions and the remaining 7% in the dissolved form in plasma.