煤烟通常是燃料不完全燃烧的产物。

Soot is usually the product of the imperfect combustion of fuel.

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燃烧的产物是水，是世界上最干净的能源。

The product of combustion is water, is the world's most clean energy.

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燃烧的产物是水，是世界上最干净的能源。

The product of combustion is water, is the world's most clean energy.

youdao