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声波在砼中的传播速度是分析桩身质量的指标之一，一般其完好桩的声速应为（）

A.≤3000m/s
B.3500m/s—4500m/s
C.2000m/s-4500m/s
D.2000m/s-3000m/s
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单项选择题
Cats are creatures of habit.They like to go to sleep about the same time every day and for a certain length of time.They seem to have a natural clock inside them that tells them when to sleep.
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Winston Churchill took cat naps.So did Presidents Harry Truman,John F.Kennedy and Lyndon B.Johnson.These famous men were known for their energy.They were also able to work long hours,often into the night.Napping was their secret.
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