What is a "speed retarder"?

1. AAn electronic system that automatically operates the brakes when travelling down a steep descent, thereby preventing overheating and loss of the brakes.
2. BAn electrical or hydraulic mechanism that is not connected to the vehicle's braking system and serves to slow the vehicle when operated.
3. CA system installed in the vehicle that assists in emergency braking and stops the vehicle on the spot.
4. DAn electrical mechanism installed in the braking system that shuts off the engine, thereby maintaining a constant speed of the vehicle on any road.

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