In（）thecontrolactionisindependenton(of)theoutput.
A. aclosedloopcontrolsystem
B. anopenloopcontrolsystem
C. aboilercontrolsystem
D. asteeringgearcontrolsystem
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Theerrorinthemeasurementofthealtitudeofacelestialbody，causedbyrefraction，increasesasthe（）.
A. horizontalparallaxdecreases
B. observer'sheightabovesealevelincreases
C. humidityoftheatmospheredecreases
D. altitudeofthebodydecreases
Usuallythepanelsareinterlockedwith（）sothatthedevicescanbemadedeadbeforethedoorsareopened.
A. protectiveearth
B. isolators
C. groundswitch
D. circuit-breakers
Thepurposeofthe“set”handonananeroidbarometeristo（）.
A. adjustthebarometer
B. indicateanychangeinthereadingofthebarometer
C. provideacorrectionforheightabovesealevel
D. provideacorrectionfortemperaturechanges
It（）foranemergencygeneratortorequireparalleling.
A. isusual
B. isnotusual
C. isnecessary
D. isnotimpossible
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