Identify the redox couples involved in the following reactions:
$(a) \ Cu + Zn^{+2} \to Cu^{+2} + Zn$
$(b) \ Mg + Fe^{+2} \to Mg^{+2} + Fe$
$(c) \ Br_{2} + 2Cl^{-} \to Cl_{2} + 2Br^{-}$
$(d) \ Fe + Cd^{+2} \to Cd + Fe^{+2}$

$A$ redox couple is defined as having together the oxidized and reduced forms of a substance taking part in an oxidation or reduction half-reaction.
$(a) \ Cu^{+2}/Cu \ \text{and} \ Zn^{+2}/Zn$
$(b) \ Mg^{+2}/Mg \ \text{and} \ Fe^{+2}/Fe$
$(c) \ Br_{2}/Br^{-} \ \text{and} \ Cl_{2}/Cl^{-}$
$(d) \ Fe^{+2}/Fe \ \text{and} \ Cd^{+2}/Cd$