单项选择题
The sub-multiple prefix “nano” (n) 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 Possible phase relationships between voltage and current in an alternating current circuit include which of the following conditions?（）
单项选择题 The resistance of a conductor varies （）
单项选择题 A switchboard for an AC electrical system requires the use of which of the following devices（）.
