On a dual air brake system, what does a low air pressure warning device do, and at approximately what pressure must it activate per FMVSS 121?

1. It releases the spring brakes and activates at 45 psi
2. It loads the compressor and activates at 85 psi
3. It warns the driver and must activate at or below 60 psi
4. It applies the service brakes and activates at 100 psi