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
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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
According to this passage, deep dream sleep is more likely to occur in ______.
A．flies
B．vulnerable birds
C．puppies
D．tigers
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