Which of the Following Mechanisms Probably Contribute to the Beneficial

Which of the following mechanisms probably contribute to the beneficial effects of continuous positive airway pressure (CPAP) in treating atelectasis?
1) Recruitment of collapsed alveoli
2) Decreased work of breathing
3) Improved distribution of ventilation
4) Increased efficiency of secretion removal

A) 1, 2, and 4 only
B) 2 and 3 only
C) 1 and 4 only
D) 1, 2, 3, and 4