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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热门试题

单项选择题

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 ever-changing environment
D．avoid being swept away by rapid currents

单项选择题

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