Which of the following properties of alveoli is/are helpful in gaseous exchange during respiration in human beings? I. The epithelial lining of alveoli is very thin. II. Elastic fibres are present in alveolar wall. III. Wall of alveoli is surrounded by many blood capillaries. Select the correct answer using the code given below.

All three features assist gaseous exchange in alveoli. (I) The single-layered, very thin squamous epithelium minimises the diffusion distance for O2 and CO2 across the respiratory membrane. (II) Elastic fibres in the alveolar wall allow expansion during inspiration and recoil during expiration, helping ventilation. (III) The dense network of pulmonary capillaries surrounding each alveolus maintains a steep concentration gradient and provides a vast surface area for gas exchange. Hence all three statements are correct.