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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