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
Real competition can happen only when _______.
A．academic standard is improved
B．there are comparable schools with different educational qualities
C．students have different interests
D．schools of all areas have sufficient budget for their development
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