Correct Option: D

Pneumatic boneis ia hollow bone filled with air, connected to the respiratory system. Many of a bird’s bones are pneumatic, with struts across their hollow interiors to provide a combination of light weight and strength as an adaptation to flying. For flight to occur in Aves, four physical forces (thrust and drag, lift and weight) must work together. In order for birds to balance these forces, certain physical characteristics are required. Flying birds have evolved reduced weight through several characteristics. Pneumatic bone is hollow or filled with air sacs, reducing weight. The loss of teeth, gonadal hypertrophy, and fusion of bones also reduce weight. Teeth have been replaced by a light weight bill made of keratin, and chewing occurs in the bird’s gizzard. Other physical characteristics required for flight are a keel for the attachment of flight muscles, an enlarged cerebellum for fine motor coordination, and a furcula, which enhances skeletal bracing for the stresses of flight.