Identify which of the following cell reactions is spontaneous under standard state conditions.

• A
  $Ca(s) + Cd^{2+}(aq) \rightarrow Ca^{2+}(aq) + Cd(s), [E^0_{Ca^{2+}/Ca} = -2.866 \text{ V}, E^0_{Cd^{2+}/Cd} = -0.403 \text{ V}]$
• B
  $2Br^-(aq) + Sn^{2+}(aq) \rightarrow Br_2(l) + Sn(s), [E^0_{Br_2/Br^-} = 1.08 \text{ V}, E^0_{Sn^{2+}/Sn} = -0.136 \text{ V}]$
• C
  $2Ag(s) + Ni^{2+}(aq) \rightarrow 2Ag^+(aq) + Ni(s), [E^0_{Ag^+/Ag} = 0.799 \text{ V}, E^0_{Ni^{2+}/Ni} = -0.257 \text{ V}]$
• D
  $2Au(s) + Zn^{2+}(aq) \rightarrow 2Au^+(aq) + Zn(s), [E^0_{Au^+/Au} = 1.68 \text{ V}, E^0_{Zn^{2+}/Zn} = -0.763 \text{ V}]$