A student demagnetises a magnetised steel bar. He puts the bar inside a solenoid that is connected to a power supply. He then takes the bar out of the solenoid. Which row shows the most effective method for demagnetising the bar?
- Atype of power supply: a.c.; speed to remove bar: fast
- Btype of power supply: a.c.; speed to remove bar: slow
- Ctype of power supply: d.c.; speed to remove bar: fast
- Dtype of power supply: d.c.; speed to remove bar: slow