Birds are warm-blooded, vertebrate animals whose bodies are covered with feathers. Other characteristics of birds are: wings; a short tail bone; the visible tail being composed entirely of feathers; a weak or nonexistent sense of smell; no teeth; and keen hearing, despite the absence of external ears. As in mammals (哺乳动物), the only other group of warm-blooded animals, the blood in the veins (静脉) of birds is kept totally separated from the blood in the arteries (动脉).
Birds evolved from reptiles, especially from some small dinosaur-relative. Gradually the scales of the dinosaur evolved into feathers, perhaps first for insulation as the animal evolved from cord to wing blade but later for flight. It is still unknown, however, whether flight developed when a ground-living form began to flap its wings to increase its running speed (like the ostrich today), or when a tree climbing form used imperfect wings to aid it in leaping from tree limb to tree limb. The evolution from reptile to the bird as we know it concluded about 75 million years ago, at the beginning of the Tertiary period, or age of mammals.
The earliest classification of birds relied upon easily recognized external characteristics or upon peculiarities of behavior or appearance. For example, all web-footed birds were grouped together and all seed-eating birds were grouped together. Since the 19th century and the general acceptance of evolution, however, this method has given way to a sounder and more scientific procedure. Birds, as well as all other living things, are now classified in a way that best expresses their blood relationships and lines of descent. This kind of classification is known as a phylogenetic classification (系谱分类).What sense is weak or nonexistent among birds（）

A．Hearing.
B．Sight.
C．Smell.
D．Tast

Birds are warm-blooded, vertebrate animals whose bodies are covered with feathers. Other characteristics of birds are: wings; a short tail bone; the visible tail being composed entirely of feathers; a weak or nonexistent sense of smell; no teeth; and keen hearing, despite the absence of external ears. As in mammals (哺乳动物), the only other group of warm-blooded animals, the blood in the veins (静脉) of birds is kept totally separated from the blood in the arteries (动脉).

B.
Birds evolved from reptiles, especially from some small dinosaur-relative. Gradually the scales of the dinosaur evolved into feathers, perhaps first for insulation as the animal evolved from cord to wing blade but later for flight. It is still unknown, however, whether flight developed when a ground-living form began to flap its wings to increase its running speed (like the ostrich today), or when a tree climbing form used imperfect wings to aid it in leaping from tree limb to tree limb. The evolution from reptile to the bird as we know it concluded about 75 million years ago, at the beginning of the Tertiary period, or age of mammals.

The earliest classification of birds relied upon easily recognized external characteristics or upon peculiarities of behavior or appearance. For example, all web-footed birds were grouped together and all seed-eating birds were grouped together. Since the 19th century and the general acceptance of evolution, however, this method has given way to a sounder and more scientific procedure. Birds, as well as all other living things, are now classified in a way that best expresses their blood relationships and lines of descent. This kind of classification is known as a phylogenetic classification (系谱分类).