I think that the excretory system is not in charge of B. This means that it does not exchange carbon dioxide with oxygen. This is a type of function that is done by the respiratory system. Do remember that the excretory system work hand in hand with the respiratory system. The other functions that are mentioned can be done by the excretory system.
A is actually important because this is the main function of the excretory system which is to remove wastes from the body. If these wastes are not removed, they will go back to other parts of the body. It may cause the blood to become poisonous.