Part II Reading Comprehension (35 minutes) Passage One Questions 21 to 25 are based on the following passage.
Birds that are literally halfasleep -- 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 halfbrain sleep in a wide range of birds. The brain hemispheres take turns sinking into the sleep stage characterized by slow brain waves. The eye controlled 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, endoftherow 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 singlehemisphere sleep, rather than total relaxation, more often than inner ducks did. Rotating 16 birds through the positions in a fourduck row, the researchers found outer birds halfasleep during some 32 percent of dozing 12 percent for birds in internal spots.
"We believe this is the first evidence for an animal behaviorally controlling sleep and wakefulness simultaneously in different regions of the brain," the researchers say.
The results provide the best evidence for a longstanding supposition that singlehemisphere sleep evolved as creatures scanned for enemies. The preference for opening an eye on the lookout side could be widespread, he predicts. He's seen it in a pair of birds dozing sidebyside in the zoo and in a single pet bird sleeping by a mirror. The mirrorside eye closed as if the reflection and the other eye stayed open.
Useful as halfsleeping might be, it's only been found in birds and such water mammals (哺乳动物) as dolphins, whales, and seals. Perhaps keeping one side of the brain awake allows a sleeping animal to surface occasionally to avoid drowning.
Studies of birds may offer unique insights into sleep. Jerome M. Siegel of the UCLA says he wonders if birds' halfbrain sleep" is just the tip of the iceberg (冰山)" He speculates that more examples may turn up when we take a closer look at other species.
21. A new study on birds' sleep has revealed that ________. A) halfbrain sleep is found in a wide variety of birds B) halfbrain sleep is characterized by slow brain waves C) birds can control their halfbrain sleep consciously D) birds seldom sleep with the whole of their brain at rest 22. According to the passage, birds often half sleep because ________. A) they have to watch out for possible attacks B) their brain hemispheres take turns to rest C) the two halves of their brain are differently structured D) they have to constantly keep an eye on their companions
23. The example of a bird sleeping in front of a mirror indicates that ________. A) the phenomenon of birds dozing in pairs is widespread B) birds prefer to sleep in pairs for the sake of security C) even an imagined companion gives the bird a sense of security D) a single pet bird enjoys seeing its own reflection in the mirror
24. While sleeping, some water mammals tend to keep half awake in order to ________. A) alert themselves to the approaching enemy B) emerge from water now and then to breathe C) be sensitive to the everchanging environment D) avoid being swept away by rapid currents
25. By "just the tip of the iceberg" (Line 2, Para. 8), Siegel suggests that________. A) half brain sleep has something to do with icy weather B) the mystery of halfbrain sleep is close to being solved C) most birds living in cold regions tend to be half sleepers D) halfbrain sleep is a phenomenon that could exist among other species
12 percent for birds in internal spots. 
and the other eye stayed open.
