单项选择题
The coil of wire rotating in the magnetic field produces the（）.
A.voltage B.current C.power D.frequency
单项选择题 （） is used to produce electric power.
单项选择题 The system will include various （） such as low tank level alarms and remote operation of tank outlet valves in the events of a fire.
单项选择题 Electric insulation is made of organic substances and so gradually （） with age.
