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

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热门试题

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

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