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.
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单项选择题
It is because of ______ that dolphins seldom sleep according to some scientists' research findings.
A．their stupidity
B．their vulnerability
C．the possible dangers in the ocean
D．their small size
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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