Oxidation of sulfur dioxide into sulfur trioxide in the absence of a catalyst is a slow process but this oxidation occurs easily in the atmosphere. Explain how does this happen.

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1. AThe atmosphere contains a high concentration of sulfur dioxide, which speeds up the oxidation process.
2. BSulfur dioxide undergoes auto-catalysis in the atmosphere, accelerating its own oxidation.
3. CAtmospheric oxygen and moisture act as natural catalysts, promoting the oxidation of sulfur dioxide.
4. DAtmospheric pressure increases the rate of oxidation of sulfur dioxide.