单项选择题
An additive used to improve the ability of a lube oil to reduce friction is known as a/an （）.
A.suppressant additive
B.dispersant additive
C.extreme pressure additive
D.viscosity improver additive
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The suction - force principle of operation is a typical characteristic of （）.
A.centrifugal pumps
B.jet pumps
C.propeller pumps
D.reciprocating pumps -
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Air leakage between the shaft and stuffing box packing in a centrifugal pump is prevented by （）.
A.a compressed packing gland
B.lantern tings between the packing rings
C.a liquid seal
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Tube leaks in the distillate cooler of a two stage flash evaporator will result in （）.
A.loss of second stage vacuum
B.contamination of the distillate
C.lower feed inlet temperature
D.contamination of the second stage condenser
