Which of the listed pumps is NOT a positive displacement pump?（）

One cause of high head pressure occurring in a refrigeration system can be （）.

If the suction pressure at the refrigeration compressor is below normal, the cause can be（）.

A gradual decrease in the discharge pressure of an operating hydraulic pump can be caused by （）.

Low compressor head pressure in a refrigeration system can be caused by（）.

When a refrigeration compressor has developed a high head pressure as a result of a refrigerant overcharge, you should （）.

The major source of chemical contaminants in hydraulic fluid is （）.

When the steering gear is in operation, you should （）.

On tank vessels equipped with power operated cargo tank valves, the type of power operator most commonly used is （）.

A salinity indicator is used to determine the （）.