Final answer:
If a driver crosses the air lines when hooking up to an old trailer, it can result in either the tractor brakes being applied instead of the trailer brakes or no trailer brakes if the trailer doesn't have spring brakes.
Step-by-step explanation:
When a driver crosses the air lines while hooking up to an old trailer, it can have several consequences depending on the type of braking system. If the trailer has spring brakes, crossing the air lines would cause the hand valve to apply the tractor brakes instead of the trailer brakes, option A. On the other hand, if the trailer does not have spring brakes, the driver could drive away but would not have trailer brakes, option C.