In an Anti-lock Braking System (ABS), how does the system prevent wheel lockup during emergency braking?

1. By rapidly modulating (releasing and reapplying) hydraulic pressure to the wheel brake calipers
2. By applying engine braking automatically using variable valve timing
3. By cutting fuel delivery to the engine cylinders to stop the vehicle power
4. By mechanically locking the brake pads to the rotor to ensure zero slip