(1). burning
(2). coal
(3). Oxygen delivered through cannulas
(4). Smoking
(5). Drinking alcohol
(6). influenza
(7). bacteria
(8). Cardiovascular exercises

Fact 1: Smoking causes direct damage to the lungs. Fact 2: Smoking causes irreversible lung damage by breaking the air sacs within the lungs. Given the two facts above, what damages air sacs?
Answer:
(4).