Final answer:
The functions of the respiratory system include helping to maintain blood pH, defense against pathogens, and enabling vocalization. Therefore the correct answer is d) all of the above
Step-by-step explanation:
The functions of the respiratory system include helping to maintain blood pH, defense against pathogens, and enabling vocalization. These functions are carried out by the major organs of the respiratory system such as the lungs, trachea, and bronchi. For example, the respiratory system helps to maintain blood pH by regulating the levels of carbonic acid in the blood. This is important because the body needs to maintain a specific acid-base balance in order for cells to function properly.