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

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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