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
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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
单项选择题
Predators are ______.
A．able to prey even when they are in deep dream sleep
B．more likely to experience dream sleep
C．incapable of preying when immobilized by dreamless sleep
D．good at preying on stupid animals
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