41. Alveoli are microscopic air sacs in the lungs of mammals. Which statement best describes how the structure of the alveoli allows the lungs to function properly? (A) They increase the amount of energy transferred from the lungs to the blood (B) They increase the flexibility of the lungs as they expand during inhalation (C) They increase the volume of the lungs, allowing more oxygen to be inhaled (D) They increase the surface area of the lungs, allowing efficient gas exchange