Some Bird Bones Are Hollow Rather Than Honeycombed

Multiple Choice

Some bird bones are hollow rather than honeycombed. The hollow bones mostly contain air sacs. The replacement of bone marrow with air sacs is properly understood as an adaptation that ________.
1) reduces the weight of the bird
2) facilitates flight
3) eliminates the functions of bone marrow

A) only 1
B) only 2
C) only 3
D) 1 and 2
E) 1, 2, and 3