What is the safety purpose of dividing a heavy vehicle pneumatic brake system into two independent circuits (dual-circuit brakes)?

1. If one circuit develops a rupture or pressure leak, the other independent circuit continues to provide sufficient braking power to stop the vehicle
2. One circuit is used exclusively for reversing, while the other is used for forward motion
3. To allow the driver to brake individual wheels manually using separate foot pedals
4. To use one circuit for fuel delivery and the second for braking