Put your skills to the test by answering this example theory test practice question.
                    
                    
                        This DVSA LORRY practice question has been taken from the Braking Systems theory test topic.
                    
                    
                        
                            Select one answer
                        
                        
                            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?