According to my experience, when u are climbing, you can put your car at D as it will shift wherever it is comfortable but it not the case when coming down the hills. You need to shift gear to 2 or L or if you keep it on D it will change the gear automatically and accelerate i.e. Dangerous. L or 2 is to increase the braking of car.
When coming down the slope, always try to control car with gears not brakes.
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