Why is the bird's lung more efficient than a human?
a. the air in the bird's lung flows in both directions
b. the air in the bird's lung is mixed with dead air
c. the air in the bird's lung flows in one direction
c. the air in the bird's lung flows in a countercurrent direction

Option C.

Explanation:

The bird's lung are said as more efficient than a human or mammals.

The bird's are having small lungs with nine air sacs playing an important role in respiration.The air sacs allows a unidirectional flow of air inside the lungs while in human the air flow takes place in bidirectional. Unidirectional flow results in the flow of fresh air along with high oxygen content in bird's lung. So, there is more oxygen available to diffuse into the blood in bird lungs.

Hence, the correct option is C.

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