What is the primary difference in friction component application between disc and drum brakes?

1. Disc brakes squeeze pads against a flat rotor; drum brakes push shoes outward against a hollow cylinder
2. Disc brakes push shoes outward; drum brakes squeeze pads inward
3. Disc brakes use hydraulic pressure; drum brakes only use air pressure
4. Disc brakes are only used on front wheels; drum brakes are only used on front wheels