In which of the following situations should you use the speed retarder when driving a bus in a mountainous terrain?

A retarder is an auxiliary braking system designed for prolonged speed reduction without the risk of the service brakes overheating and losing effectiveness. In mountainous terrain, on long descents, its use is crucial for maintaining safety, as it allows the service brakes to be kept ready for emergency braking. Regular use of the retarder prevents the dangerous phenomenon of 'fading', which is the loss of braking force due to excessive temperature.