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

The amount of fluid friction or internal friction inherent in a mineral lubricating oil is reflected as a measure of its （）.

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

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

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

The control mode in which the final control element is moved from one of two flexed positions to the other is known as （）.

An increasing head pressure in a refrigeration system, without any corresponding change in the cooling water inlet temperature, would probably be caused by （）.

Which of the following problems could cause the high pressure cutout switch to shut down the compressor in a refrigeration system?（）

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

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