单项选择题

Compare with dreamless sleep, deep dream sleep is _______.

A．not the result of natural selection
B．less likely to appear to moneys
C．more protective to the animals
D．at a higher stage of evolution
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单项选择题
According to some scientists' research findings, dolphins seldom sleep because _______.
A．of their stupidity
B．of their vulnerability
C．there are possible dangers in the ocean
D．aquatic mammals do not need sleep
单项选择题
The example of dogs and cats in the second paragraph is intended to _______.
A．explain which animals are mammals
B．show the differences between mammals
C．illustrate how shallow dreamless sleep is
D．reveal how smart they are
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