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which of the following is true regarding respiration in birds?
a) it is inefficient because blood is pumped to the lungs under very low pressure.
b) it is inefficient because oxygen-poor air mixes with oxygen-rich air during inhalation.
c) it is highly efficient, as a bird’s circulatory system includes three circuits.
d) it is highly efficient, as oxygen-rich air enters the lungs even while a bird is exhaling.

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.

Hence, option D is correct.