Option D, It is highly efficient, as oxygen-rich air enters the lungs even while a bird is exhaling
Explanation:
In the respiratory system of birds, air flows only in one direction due to which a bird is capable to take in oxygen even when it is exhaling. The air sacs allow only unidirectional flow of air, which is more fresh and contains high oxygen content. Even a bird’s lung are capable to store and pump air, when they are at stationary position.
