Which Reaction Accounts for the Fact That the PH of Rain

Multiple Choice

Which reaction accounts for the fact that the pH of rain is naturally slightly acidic?

A) CO₂ + H₂O $\rarr$ H⁺ + HCO₃¯
B) SO₃ + H₂O $\rarr$ 2 H⁺ + SO₄^2¯
C) Ca²⁺ + CO₂ + H₂O $\rarr$ CaCO₃ + 2 H⁺
D) NO₂ + H₂O $\rarr$ 2 H⁺ + NO₃¯