Which of the following organs is most affected by a deficiency of $O_2$ in humans?

At what $pCO_2$ level may death occur?

$A$: Emphysema is a chronic obstructive pulmonary disease,causing irreversible distension and loss of elasticity of the alveoli.
$R$: Emphysema is preventable if chronic exposure to smoke (cigarette and others) and pollutants is avoided.

Give differences between Asthma and Emphysema.

In this disease,the respiratory surface area decreases.

Assertion $A$: Pneumonia is a serious respiratory disease.
Reason $R$: The accumulation of fluid in the alveoli and bronchioles prevents sufficient oxygen from reaching the lungs.
Which option is correct for Assertion $A$ and Reason $R$?