[单项选择]

Birds that are literally half-asleep---with one brain hemisphere alert and the other sleeping- control which side of the brain remains awake, according to a new study of sleeping ducks.
Earlier studies have documented half-brain sleep in a wide range of birds. The brain hemi- spheres take turns sinking into the sleep stage characterized by slow brain waves. The eye con- trolled by the sleeping hemisphere keeps shut, while the wakeful hemisphere’ s eye stays open and alert. Birds also can sleep with both hemispheres resting at once.
Decades of studies of bird flocks led researchers to predict extra alertness in the more vulnerable, end-or-the-row sleepers. Sure enough, the end birds tended to watch carefully on the side away from their companions. Ducks in the inner spots showed no preference for gaze direction.
Also, birds dozing(打盹) at the end of the line resorted to single-hemisphere sleep, rather than total relaxation, more often than inner ducks
A. half-brain sleep has something to do with icy weather
B. the mystery of half-brain sleep is close to being solved
C. most birds living in cold regions tend to be half sleepers
D. half-brain sleep is a phenomenon that could exist among other species