What is the purpose of the extendable stop arm on the side of a school bus?

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1. It provides an additional, highly visible signal to traffic that the bus is stopped and students may be crossing
2. It is used exclusively to indicate the bus is experiencing a mechanical problem
3. It functions only as a physical barrier that stops students from stepping into the road, not as a signal to traffic
4. It replaces the need for any of the bus's warning lights

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