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
单项选择题 The sub-multiple prefix “micro” means （）.
单项选择题 A signal derived from a controlled function and returned to the initiating point is called a/an （）.
单项选择题 The purpose of a main switchboard circuit breakers reverse-power trip is to （）.
