Of course the presence of humans themselves also changes the odds of fire, as people are a frequent source of ignition—both accidental and deliberate.

当然，人类自身的存在也改变火灾的可能性，因为人类是一个经常地导火源——有无意的也有故意的。

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NO2 is the main source of nitrate aerosols, which form an important fraction of PM2.5 and, in the presence of ultraviolet light, of ozone.

二氧化氮是硝酸盐气溶胶的主要来源，构成PM 2.5和在紫外线下臭氧的主要部分。

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The presence of hydrogen in the soil, meanwhile, could provide a source of fuel for lunar visitors.

在土壤中存在的氢，将为月球的探访者带来燃料。

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Subor, an actor with an admirable way with words, is given little to say and little to do, though his brooding presence is itself a source of fascination.

NEWYORKER: The Intruder

Investigators are going into Tsarnaev's room every few hours to ask questions in the presence of doctors, the source said.

CNN: Boston bomb suspect: My brother was the mastermind

Unlike existing works that emphasise institutional and historical factors, this study analyses the importance of the presence of fixed costs as a source of market imperfections and very high profits in certain towns.

FORBES: The Market for Lemons and the Start of the Mafia