What is the purpose of the supply (wet) tank in an air brake system?

1. To receive compressed air directly from the compressor and collect the initial condensed water and oil before distributing clean air to primary and secondary service tanks
2. To hold a reserve of air dedicated solely to the parking brakes
3. To act as the final tank supplying the trailer service line
4. To store air at a higher pressure than the rest of the system for emergency stops