How is good (stratospheric) ozone formed?

Stratospheric ozone (O₃) forms when a single oxygen atom (O) combines with an oxygen molecule (O₂). The result is a molecule with three oxygen atoms, O₃, so the correct choice is “combining a single oxygen atom (O) with an oxygen molecule (O₂).” Breaking apart O₂ can help produce the oxygen atom, but it does not form ozone by itself.

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