What indicates loss of air when disconnecting the control line from the tractor?

Answer: B — Supply air stops immediately

When disconnecting the control line from the tractor, a loss of air is indicated when the supply air stops immediately. This happens because the supply line is what provides the air pressure necessary for the functioning of the braking system. If you disconnect this line, the air pressure supply would be cut off instantly, demonstrating that the system is no longer receiving air to maintain the necessary pressure for braking. Understanding this concept is critical because it highlights how air brake systems operate and the importance of maintaining proper connections for safety. The immediate cessation of supply air confirms that the system relies on this input to function effectively, and any interruption can have significant implications for vehicle safety and braking efficiency.