单项选择题

Compare with dreamless sleep, deep dream sleep is ______.

A．not the result of natural selection
B．less likely to appear to moneys
C．more protective to the animals
D．at a higher stage of evolution
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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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