What are forest fires often caused by?

森林大火经常是由什么引起的？

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Forest fires are often caused by broken glass or cigarette ends.

森林火灾时常由碎玻璃或香烟头引起。

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Forest fires are often caused by broken glass or cigarette ends.

森林火灾时常由碎玻璃或香烟头引起。

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