Final answer:
Chronic lower respiratory diseases are the third leading cause of death in the United States, with smoking being the major cause of these conditions.
Step-by-step explanation:
The third leading cause of death in the United States is chronic lower respiratory diseases (CLRD), which include chronic obstructive pulmonary disease (COPD), emphysema, and chronic bronchitis, among others. These diseases often result from long-term exposure to irritating gases or particulate matter, most frequently from cigarette smoke.
Smoking is the major cause of chronic respiratory disease, and exposure to tobacco smoke is the leading cause of preventable death in the United States. In contrast, other diseases listed in the multiple-choice options, like influenza and pneumonia, although significant, do not constitute the third leading cause of death in the U.S.