D will hit last because if air resistance is null then the only force enacting on the balls is the force of gravity. Force of Gravity has an acceleration of 9.81 ms^2 and so every ball had the same acceleration and ball D is the furthest away from the floor.
Answer
When an electric current flows in a wire, it creates a magnetic field around the wire. This effect can be used to make an electromagnet. A simple electromagnet comprises a length of wire turned into a coil and connected to a battery or power supply.
A coil of insulated wire is wrapped around an iron nail. One end of the coil of wire is connected to a battery. The other is connected to a switch in the circuit
A simple electromagnet
You can make an electromagnet stronger by doing these things:
wrapping the coil around a piece of iron (such as an iron nail)
adding more turns to the coil
increasing the current flowing through the coil
There is a limit to how much current can be passed safely through the wire because the resistance of the wire causes heating.
The second one because energy is being covered around in a circle causing electricity to flow
