What is the danger in changing gear while driving when driving a heavy vehicle down a steep descent?

1. AThere is no danger in changing gear and it does not matter which gear is engaged, as long as one of them is engaged.
2. BThere is no danger in changing gears on a steep descent.
3. CThere is a danger that the gear will not engage and the vehicle will remain in neutral.
4. DThere is no danger: changing from a low gear to a high gear on a steep descent will allow the vehicle's speed to increase.

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