单项选择题
A semiconductor is a material with a （）.
A.conductivity higher than a normal conductor
B.conductivity higher than a normal insulator
C.high conductivity at low temperatures
D.low conductivity at high temperatures
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The sub-multiple prefix “micro” means （）.
A.thousandth (10 to the-3rd power)
B.millionth (10 to the-6th power)
C.billionth (10 to the-9th power)
D.trillionth (10 to the-12th power) -
单项选择题
A signal derived from a controlled function and returned to the initiating point is called a/an （）.
A.monitoring signal
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The purpose of a main switchboard circuit breakers reverse-power trip is to （）.
A.prevent main circuit overload
B.protect the circuit breaker blowout coil
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D.prevent low voltage tripout
