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11. A new study on birds sleep has revealed that ____________. A) half-brain sleep is found in a wide variety of birds B) half-brain sleep is characterized by slow brain waves C) birds can control their half-brain sleep consciously D) birds seldom sleep with the whole of their brain at rest 12. According to the passage, birds often half sleep because ______________. A) they have to watch out for possible attacks B) their brain hemisphere take turns to rest 跨段 C) the two halves of their brain are differently structured 没 D) they have to constantly keep an eye on their companions 反 13. 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 14. 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 | 
