What is the last step involved in respiration?

Statement-$I$: The anatomical setup of the lungs in the thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung cavity.
Statement-$II$: The volume of air remaining in the lungs even after a forcible expiration is called residual volume.

At sea level,the blood of individuals contains approximately $5$ million red blood cells per cubic millimeter. However,individuals living at an altitude of $5400$ meters have $8$ million red blood cells per cubic millimeter. Why does this increase occur?

Aerobic respiration involves
$I.$ external respiration
$II.$ transport of gases
$III.$ internal respiration
$IV.$ cellular respiration
Choose the correct combination of options for the given statements

Which of the following statement$(s)$ is/are correct?
$(a)$ High concentration of carbonic anhydrase in RBCs.
$(b)$ High concentration of carbonic anhydrase in blood plasma.
$(c)$ Every $100 \ ml$ of deoxygenated blood delivers approximately $4 \ ml$ of $CO_2$ to the alveoli.
$(d)$ $20-25\%$ of $CO_2$ is carried by hemoglobin as oxyhemoglobin.

Which of the following statements correctly describes the respiratory processes occurring in the lungs?