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