地铁的特点有哪些？
A. 运营能力强
B. 施工简便快捷成本低
C. 施工成本较高
D. 施工专业复杂 ，涉及主体多
Infixeddisplacementvanepump,
A. flowrateisconstantanddoesnotchangewithworkingpressure.
B. flowratedecreaseswithworkingpressurerising.
C. flowrateincreaseswithworkingpressurerising.
D. noneoftheabove
Whatistheadvantageofvanepumps?
A. smoothrunning
B. lowpulsationofoutput
C. quietoperation
D. alloftheabove
Balancedvanepumpsaredesignedtohave
A. fixeddisplacement
B. variabledisplacement
C. bothfixedandvariabledisplacement
D. noneoftheabove
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