What is the purpose of the main ignition key switch ground connection in an underbone motorcycle electrical system?

1. It provides the ground path to short out the CDI ignition coil, turning the engine off when the key is turned to 'OFF'.
2. It charges the battery using ground current when the motorcycle is parked.
3. It connects the headlights directly to the engine case to reduce wiring resistance.
4. It acts as a safety fuse that blows if the key is turned too hard.