The output volume of a positive fixed displacement pump can be changed only by （）.

The general purpose of a heat exchanger is to （）.

Pitting in the suction areas of a centrifugal pump bronze impeller is usually caused by （）.

A grease gun is generally used to lubricate a （）.

Thermostatic expansion valves can be adjusted for （）.

A salinity indicator is used to determine the （）.

If the relief valve on the discharge side of a hydraulic pump lifts, the cause could be （）.

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

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

Heavy wear in a particular area of the inner circumference of a packing ring may be caused by （）.