Final answer:
Crossing the air lines can cause issues with the braking system of a trailer with potentially dangerous consequences, such as the tractor braking instead of the trailer or the absence of trailer brakes if the trailer has no spring brakes.
Step-by-step explanation:
When a driver crosses the air lines while hooking up to an old trailer, it can result in incorrect operation of the braking system. Option A, the hand valve applying the tractor brakes instead of the trailer brakes, can occur because the service and supply lines are crossed. In the case of Option B, if the trailer lacks spring brakes, driving away is possible, but without functional trailer brakes, which is dangerous. Option C, regarding brake lights, is unrelated to crossing the air lines as they are typically connected to a separate electrical circuit.
The air system in a trailer uses pneumatic pressure to operate the brakes, and as highlighted in the provided reference, air is compressible and must be properly managed within the system. The discussion about the air in the braking system needing to be bled relates to ensuring that the air lines are free of gas that can compress, which would otherwise reduce the efficiency of the brake application.