Đáp án đúng: B Dung dịch Na3PO4. $\text{C}{{\text{a}}^{\text{2+}}}\text{,}\,\text{M}{{\text{g}}^{\text{2+}}}\,\xrightarrow{\text{PO}_{\text{4}}^{\text{3-}}}\,\text{C}{{\text{a}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\text{,}\,\text{M}{{\text{g}}_{\text{3}}}{{\text{(P}{{\text{O}}_{\text{4}}}\text{)}}_{\text{2}}}\downarrow $