In the reaction $Cu_{(s)} + 2Ag^{+}_{(aq)} \to Cu^{2+}_{(aq)} + 2Ag_{(s)}$,the reduction half-cell reaction is:

• A
  $Cu + 2e^- \to Cu^{2-}$
• B
  $Cu - 2e^- \to Cu^{2+}$
• C
  $Ag^{+} + e^- \to Ag$
• D
  $Ag - e^- \to Ag^{+}$