单项选择题

By "is just the tip of the iceberg" in the eighth paragraph, Siegel suggests that ______.

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
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单项选择题
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
单项选择题
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
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