What does it mean if the low air warning light comes on while you are driving?

1. The compressor has reached its cut-out pressure and is in the unloaded cycle
2. The air system pressure has dropped below 60 psi (414 kPa) and you must bring the vehicle to a safe stop immediately
3. The automatic slack adjusters are performing a self-correction adjustment
4. The trailer gladhand couplings have become crossed