单项选择题
If a centrifugal pump is driven by a constant speed electric motor and produces a discharge pressure less than designed, the cause is probably（）.
A.excessive pump speed
B.excessive suction head
C.worn wearing rings
D.gross pump misalignment
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A.Distillation is the process of boiling seawater to produce vapor which is condensed into fresh water
B.Evaporation is the second part of the distillation process where brine is removed
C.Brine is the result of condensed sea water vapor
D.Distillate is the product resulting from the evaporation of fresh water vapor -
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The amount of moisture in a given sample of air, when compared with the amount of moisture the air could hold if totally saturated at the existing temperature of the sample, is called （）.
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The purpose of an air compressor unloading device is to （）
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