Why must a two-stroke motorcycle engine that lacks a separate oil sump have oil mixed with the petrol or supplied through an oil-injection system?

1. Because two-stroke engines do not need lubrication at all
2. Because the crankcase in most two-stroke engines is also used to pre-compress the air-fuel mixture, so a wet sump of oil cannot be kept there, yet moving parts still need lubrication
3. Because mixing oil with petrol increases the octane rating of the fuel
4. Because oil mixed with petrol prevents the spark plug from fouling