填空题 Look at the four squares, , , , and , which indicate where the following sentence could be added to the passage. Where would the sentence best fit?
This irritation is caused by viruses or bacteria, exposure to tobacco smoke, or air pollution.
Bronchitis is an inflammation of the bronchial tubes in the lungs. It often appears after a cold or an upper respiratory infection that does not heal completely. It also may accompany childhood infections such as measles, whooping cough, and typhoid fever. The inflamed bronchial tubes secrete a sticky mucus called sputum. It is difficult for the tiny hairs on the bronchi to clear out this sputum. The cough that comes with bronchitis is the body's attempt to eliminate it.