The birds not have respiratory trouble at the time of flying at high altitude. What is the reason for this?

Question:The birds not have respiratory trouble at the time of flying at high altitude. What is the reason for this?

1.The size of lungs of birds is larger in comparison to their body

2.At higher altitudes birds fly inactively

3.Birds have extra air sacs

4.None of the above is a correct reason

Answer:3