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

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

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

Which of the following statements is correct concerning heat transfer?（）

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

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

An excessive charge of refrigerant in an air-cooled refrigeration system can cause （）.

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

Which of the problems listed would came the discharge temperature of an R-12 refrigeration compressor to increase?（）

A pressure drop through the refrigerant liquid line may cause （）.