What is the primary function of the Air Dryer (Lufttrockner) in a commercial vehicle's pneumatic brake system?

1. To cool the truck cabin on hot summer days
2. To remove moisture and oil vapor from the compressed air before it enters the air reservoirs and valves, preventing internal corrosion and winter freezing of brake valves
3. To blow warm air onto the front windshield wipers
4. To dry the driver's clothing in rainy weather