Your tractor unit has three air lines. You're connecting to a trailer that has two air-line couplings. What colour is the line you shouldn't connect to the trailer? Your tractor unit has three air lines. You're connecting to a trailer that has two air-line couplings. What colour is the line you shouldn't connect to the trailer?

• Possible answers:

   
• A. Red
• B. Yellow
• C. Black
• D. Blue