單項(xiàng)選擇題Operational amplifiers, used primarily in analog circuits, are characterized by（）

A.high input impedance, high gain and low output impedance
B.high input impedance, high gain and high output impedance
C.low input impedance, low gain and high output impedance
D.low input impedance, high gain and low output impedance


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1.單項(xiàng)選擇題CFC refrigerants exposed to high temperature or direct flame, will decompose and may produce（）

A.methyl chloride
B.ammonia
C.hydrofluoric acid
D.ozone

2.單項(xiàng)選擇題The major difference between the discharge and suction valves installed in most low pressure, reciprocating air compressors is that（）

A.one valve seats upwards, while the other seats downwards
B.the reed valves used on the discharge are made substantially thicker and heavier than the suction valves
C.the suction valve springs exert a greater tension than the discharge valve springs
D.the discharge valve springs exert a greater tension than the suction valve springs

3.單項(xiàng)選擇題Which of the following problems will occur if the internal drain at either end of a hydraulic, two-way, spool-type directional control valve cylinder were to become plugged?（）

A.The reservoir would become vapor bound
B.The valve would be placed in hydraulic lock
C.The spring loaded relief ports would open
D.The buffering chambers would be unable to function

4.單項(xiàng)選擇題Hydraulically, servo-operated, automatic, change over valves, utilized in a two ram hydraulic steering gear, serve to（）

A.allow an alternate main pump to start in the fully loaded condition thus developing immediate full torque
B.prevent either main pump from being hydraulically motored when idle by cross pressure flow
C.prevent both units from operating simultaneously which could result in doubling the flow of oil and pressure leading to over pressurization of the system
D.all of the above

5.單項(xiàng)選擇題Although lube oils used in the main lubricating service systems should have a relatively high flash point to avoid ignition, they can create smoke and fire hazards when they（）

A.are exposed to a vacuum
B.come in contact with extremely hot surfaces
C.are reduced in temperature to just above the pour point
D.become extremely agitated or aerated