Propeller pitch speed minus ship speed divided by the propeller pitch speed is termed （）.

A.two or more impellers housed together in one casing and mounted on a single shaft
B.a single stepped impeller mounted in a progressively staged casing
C.a large radial clearance between the impeller and casing to prevent overheating due to
D.an increase in the discharge velocity of the liquid with a corresponding decrease in pressure through the stages

A.add refrigerant to the system
B.adjust the thermal expansion valve to the proper setting
C.adjust the float valve to the proper holding pressure
D.open the hand expansion valve

A.a stuck solenoid valve
B.a shortage of refrigerant
C.too much oil in circulation
D.too much superheat