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

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

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

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

If the head pressure of a reciprocating refrigeration compressor is excessive, （）.

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

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

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

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

An internal bypass is provided on some hydraulic system suction strainers to help reduce the possibility of （）.