港口与航道工程计量标准规定：当开工前测量计算确定的土石方量与设计的土石方量的差值小于或等于()时，应以设计的土石方量为准。
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离心泵是叶片泵的一种，由于这种泵的工作是靠叶轮（）时叶片拨动液体旋转，使液体获得离心力而完成水泵的输水过程。
In the past few decades,the influence of political parties has been declining（）
A. so the political parties are no longer important
B. so an independent presidential candidate can win a national election
C. but without the backing of a political party,a presidential candidate still cannot win
D. so the presidential election has become more unpredictable
钻进时悬重下降是溢流的早期。
A. 正确
B. 错误
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