Birds Are Able to Sustain Long Flights at High Altitudes

Birds are able to sustain long flights at high altitudes because they

A) can extract oxygen during both inhalation and exhalation.
B) decrease their heart rate during flight.
C) are able to lower their body temperature during flight.
D) have thicker blood than other animals, with many more red blood cells.
E) use anaerobic metabolism during flight.