Positive displacement, helical gear pumps are well suited for pumping oil because
A. stuffing boxes eliminate the leakage problems usually associated with other gear pumps
B. it is not necessary to closely maintain design clearances with this pump
C. they are essentially self-priming and produce a high suction lift
D. these pumps are designed with extreme tooth angles
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信用卡、借记卡和电子支票________。
A. 都是货币
B. 不是货币
C. 有时候是货币，取决于他们如何被使用
D. 有时候是货币，取决于他们购买什么东西
E. 有时候是货币，取决于不同的货币测定指标
A controllable pitch propeller on a diesel driven vessel eliminates the need for
A. friction clutches
B. disconnect clutches
C. reversing gears
D. reduction gears
如果某人在树下埋了一罐货币，那么货币的职能是________。
A. 交易媒介
B. 计价单位
C. 支付手段
D. 价值储藏
E. 实物交易工具
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