Which of the following factors increases the resistance of the airways?

Inhalation of cigarette smoke causes reflex constriction of airway smooth muscle as a result of stimulation of irritant receptors in the airway wall. The other choices are incorrect: Both increasing lung volume above FRC and sympathetic stimulation of airway smooth muscle reduce airway resistance. Going to high altitude does the same because the density of the air is reduced. The density is also decreased when nitrogen is replaced by helium in the inspired gas.