The answer is that Epithelial cells act as a passageway in which the air moves into the lungs. The epithelial cells in the respiratory system would serve as a leak proof barrier that do transports, filters as well as conditions the air one is breathing.
A. Muscle Fitness
I believe this is it.
I think (C):600
Explained: other sources say there is about 650 but if you count the muscles within a complex muscle then about 840