Air rushes into the lungs of humans during inhalation because A) the rib muscles and diaphragm contract, increasing the lung volume. B) the volume of the alveoli increases as smooth muscles contract. C) gas flows from a region of lower pressure to a region of higher pressure. D) pulmonary muscles contract and pull on the outer surface of the lungs. E) a positive respiratory pressure is created when the diaphragm relaxes.