Which period 3 oxide, when added to water, forms an acidic solution?

A.     \({\text{S}}{{\text{O}}_{\text{3}}}\)

B.     MgO

C.     \({\text{N}}{{\text{a}}_{\text{2}}}{\text{O}}\)

D.     \({\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}\)

A