An investigator is studying the clearance of respiratory particles in healthy non-smokers. An aerosol containing radio-labeled particles that are small enough to reach the alveoli is administered to the subjects via a non-rebreather mask. A gamma scanner is then used to evaluate the rate of particle clearance from the lungs. The primary mechanism of particle clearance most likely involves which of the following cell types? Macrophages Neutrophils Type I pneumocytes Type Il pneumocytes Goblet cells