What is the indication that air is flowing to all trailers?

The correct choice indicates a practical method of verifying that air pressure is effectively reaching all trailers in a multi-trailer setup. By opening the service line of the middle trailer, you can observe whether air is present and flowing through the system. This method directly checks if the service line is operational, allowing air to flow and confirming that the trailer is receiving the necessary air pressure for braking and other functions.

In contrast, other options may not provide as definitive a result. Listening for air noise could suggest that air is present, but it does not confirm that it is flowing adequately through the system. Checking the pressure gauge could indicate overall system pressure but doesn’t directly confirm flow to each trailer. Visual inspection of the air lines can help identify physical problems, but it does not provide conclusive evidence about airflow directly. Therefore, opening the service line serves as the most reliable method for ensuring that air is indeed flowing to all trailers.