What Is the Most Likely Reason Why Smokers Are More

What is the most likely reason why smokers are more at risk for respiratory system infections?

A) Cigarette smoke is carcinogenic (cancer-causing) ,leading to a much higher incidence of lung cancer.
B) They aren't-this is just a rumor used to get people to stop smoking.
C) Chemicals in cigarette smoke can impair the mucociliary escalator,preventing natural cleansing of the respiratory tract.
D) Smokers take in microbes from their hands into their lungs as they handle cigarettes and inhale the smoke.