which belongs to each sentence?
In [external/ or cellular] respiration, thesegases are exchanged between the body's tissues and the outsideenvironment by way of the respiratory system. In [external/ or cellular] respiration, these gases are usedand created by the metabolic processes of the cells.
[alveolar capillary membranes, type 2 alveolar cells,type 1 alveolar cells] make up the main structure of thealveolar wall. The walls of the alveoli  are very thin,separated from the capillary vessels by an even thinner barriercalled [alveolar capillary membranes, type 2 alveolarcells, type 1 alveolar cells] . [alveolar capillary membranes, type2 alveolar cells, type 1 alveolar cells] occur on theinside surface of the alveoli and secrete liquids called pulmonarysurfactants, to help the surface of the alveoli expand.
Which of the following is a general adaptation of therespiratory system that increases the rate ofdiffusion?
a. Decreasing the concentration gradient across the respiratorysurface
b. Having a small respiratory surface area
c. Having a thin respiratory surface
