Which of the following braking techniques should you use to considerably decrease the speed of an agricultural tractor driven on a very slippery surface?

Engine braking, i.e., changing down gears, allows for a gradual and controlled reduction in speed without the risk of the wheels suddenly locking. On a very slippery surface, use of the service brake could lead to a loss of grip and result in a skid. This is therefore the safest method of reducing speed in difficult road conditions, especially for a heavy vehicle.