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Given the cell reaction:$\newline$$$\mathrm{Ca}_{(\mathrm{s})}+\mathrm{Mg}^{2+}{ }_{(\mathrm{aq})} \leftrightarrows \mathrm{Ca}^{2+}{ }_{(\mathrm{aq})}+\mathrm{Mg}_{(\mathrm{s})}$$$\newline$Which substance is oxidized?$\newline$($1$) $\mathrm{Ca}_{(s)}$$\newline$($2$) $\mathrm{Mg}^{2+}{ }_{(a q)}$$\newline$($3$) $\mathrm{Ca}^{2+}{ }_{(a q)}$$\newline$($4$) $\mathrm{Mg}_{(\mathrm{s})}$$\newline$$1$$\newline$$2$$\newline$$3$$\newline$$4$