On modern ships, centrifugal pump with a self-primer has replaced reciprocating pump as bilge pump because（）
A. centrifugal pump is cheaper than reciprocating pump
B. the size of centrifugal pump with same capacity is smaller than that of reciprocating pump
C. centrifugal pump makes maintenance easier than reciprocating pump does
D. all of the above
在消费成功后若持卡人或收银员，发现消费金额有误或其他情况，此时收银员可以执行（）操作。
A. 一次刷卡消费
B. 执行撤销交易
C. 执行签退操作
D. 执行结算操
中组发[2008]10号文件对各地各部门加强老干部活动中心、老干部大学基础设施建设的要求是什么？
什么是等速过滤？什么是变速过滤？为什么说等速过滤实质上是变速的，而变速过滤却接近等速？
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