The normal rain water is slightly acidic and its pH value is 5.6 because of which one of the following ?

A. CO₂ + H₂O → H₂CO₃
B. 4CO₂ + H₂O → H₂CO₃
C. 2SO₂ + H₂O → H₂CO₃
D. CO₂ + H₂O → 2HCO₃