\text { 9. In the reaction equation } \mathrm{Cu}_{(s)}+2 \mathrm{Ag}_{\text {(aq) }}^{+} \rightarrow 2 \mathrm{Ag}_{(\mathrm{s})}+\mathrm{Cu}^{2+} \text { (aq), the correct oxidation half-reaction is } \text { (a) } \mathrm{Cu}_{(s)} \rightarrow

\mathrm{Cu}^{2+}{ }_{(\mathrm{aq})}+2 \mathrm{e}^{-} \text { (b) } \mathrm{Cu}^{2+}{ }_{(\mathrm{aq})}+2 \mathrm{e}^{-} \rightarrow \mathrm{Cu}_{[\mathrm{s})} \text { (c) } \mathrm{Ag}_{(\mathrm{aq})}+\mathrm{e}^{-} \rightarrow \mathrm{Ag}_{(\mathrm{s})} \text { (d) } \mathrm{Ag}_{(s)} \rightarrow \mathrm{Ag}_{(\mathrm{aq})}+\mathrm{e}