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
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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
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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