In emphysema
A. pressure in alveoli increases and their walls burst, coalescing them and decreasing the surface area for gas exchange.
B. cells in the bronchial lining divide too fast.
C. tar from cigarettes blackens alveoli.
D. the respiratory system gradually fails due to too little mucus and thinning of the bronchial lining.
E. membranes form within alveoli, increasing surface area.