單項(xiàng)選擇題Possible phase relationships between voltage and current in an alternating current circuit include which of the following conditions?（）

A.Current and voltage may be in phase
B.Current may lead the voltage
C.Current may lag the voltage
D.All of the above


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1.單項(xiàng)選擇題The resistance of a conductor varies （）

A.directly as its length and inversely as its cross-sectional area
B.inversely as its length and directly as its cross-sectional area
C.directly as its length and directly as its cross-sectional area
D.inversely as its length and inversely as its cross-sectional area

2.單項(xiàng)選擇題A switchboard for an AC electrical system requires the use of which of the following devices（）.

A.Frequency meter
B.Ohmmeter
C.Induction voltage regulator
D.Current transformer governor

3.單項(xiàng)選擇題The part of the shipboard electrical system used to control the distribution of power to the branch circuits, is the （）.

A.bridge control panel
B.disconnect links
C.governor relay box
D.main switchboard

4.單項(xiàng)選擇題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

5.單項(xiàng)選擇題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)