Elements X and Y both belong to Period 3 of the Periodic Table. Element X is either phosphorus or sulfur. Element Y is either sodium or magnesium. The oxide of element X reacts with water to form a solution that contains the aqueous anion $\text{XO}_4^{2-}$. One mole of element Y combines with one mole of chlorine molecules. By the end of the reaction, every particle of element Y and every chlorine molecule has been completely consumed. Which elements are X and Y?
- AX: phosphorus; Y: sodium
- BX: phosphorus; Y: magnesium
- CX: sulfur; Y: sodium
- DX: sulfur; Y: magnesium