Which of these characteristics is associated with flight requirements in birds?

A) light, hollow bones
B) a light, horny beak with no teeth
C) respiratory air sacs and lungs with one-way air flow
D) an enlarged breastbone with strong muscles attached
E) All of the answers list requirements for flight in birds.

All of the characteristics listed are requirements for flight in birds. The light, hollow bones reduce the bird's weight, making it easier to fly. The beak without teeth is also lighter, and it helps the bird eat quickly in flight. The respiratory air sacs and lungs with one-way air flow allow for a continuous supply of oxygen during flight, and the enlarged breastbone with strong muscles attached provides the necessary power for the wings to beat and generate lift.