Answer: Option A.
The bird respiratory system does not mix exhaled air with inhaled air.
Step-by-step explanation:
The respiratory system of birds are more efficient than that if humans because The bird respiratory system does not mix exhaled air with inhaled air and this is due to the fact that their lungs are u directionally ventilated instead of bidirectional flow of other organisms. The lungs undirectional is as a result of inflation and deflation of air sacs in a sequence which make the exhaled air not to mix with inhaled air.