Elements X and Y both belong to Period 3 of the Periodic Table. X is either phosphorus or sulfur, and Y is either sodium or magnesium. An oxide of X reacts with water to produce a solution that contains the aqueous anion $\text{XO}_4^{2-}$. 1 mole of element Y reacts with 1 mole of chlorine molecules. By the end of the reaction, every particle of element Y and every chlorine molecule has been completely used. What are elements X and Y?
- AX: phosphorus; Y: sodium
- BX: phosphorus; Y: magnesium
- CX: sulfur; Y: sodium
- DX: sulfur; Y: magnesium