Final answer:
At functional residual capacity, the elastic recoil of the chest wall and lungs are equal and opposite. This balance keeps the lungs open without any airflow.
Step-by-step explanation:
At functional residual capacity (end of quiet expiration) the elastic recoil of the chest wall is equal in magnitude but opposite in direction to the elastic recoil of the lungs. The correct option is a) Elastic, equal, elastic. The lungs and chest wall are elastic structures that work in opposition to each other. The lungs have an elastic recoil property that drives the lung tissue inward, while the elastic work of the intercostal muscles, chest wall, and diaphragm is directed outward. At functional residual capacity, the outward force of the chest wall is balanced by the inward pull of the lungs, thereby keeping the lungs open but without any air flow.