Which of the Following "Layers" Must Be Traversed by Gases

Which of the following "layers" must be traversed by gases moving across the alveolar-capillary membrane?
1) Alveolar epithelial membrane
2) Capillary endothelial membrane
3) Interstitial space
4) Transbronchial radial tethering mechanisms

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