Here's why:

* Reaction: Sulfur (S) reacts with oxygen (O₂) in the air.

* Product: The primary product of this reaction is sulfur dioxide (SO₂).

* Equation: The balanced chemical equation is: S + O₂ → SO₂

While it's possible for sulfur to form sulfur trioxide (SO₃) under certain conditions, this usually requires a catalyst and higher temperatures. The primary product when sulfur burns in air is sulfur dioxide.