Answer:the possession of feathers
Explanation:birds are warm blooded animals , that possess feathers.these feathers cover their bodies and used for flight and warmth.their forelimbs are modified into wings,which fold into a Z shape at rest.feathers grow from the skin follicles and are replaced by scales on their legs.they possess oil gland to dress the feathers.
Other features of birds include
1) beaks with no teeth
2) fully ossified bones with air cavities
3)females with left ovary and oviducts only
Sink holes and crevasses might be a result of this <span />
Answer - A. Deep because there is less O2 exchange
Reasoning - Bacteria has been known to only use half a percentage which is .5 percent of O2.
