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
Which of the following might be the best title for this passage
A．Evolution of Sleep.
B．Two Types of Sleep.
C．The Original Function of Sleep.
D．Animals and Sleep.
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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