Which of the following reactions is used to make a fuel cell?

• A
  $Cd_{(s)} + 2Ni(OH)_{3(s)} \to CdO_{(s)} + 2Ni(OH)_{2(s)} + H_2O_{(l)}$
• B
  $Pb_{(s)} + PbO_{2(s)} + 2H_2SO_{4(aq)} \to 2PbSO_{4(s)} + 2H_2O_{(l)}$
• C
  $2H_{2(g)} + O_{2(g)} \to 2H_2O_{(l)}$
• D
  $2Fe_{(s)} + O_{2(g)} + 4H^{+}_{(aq)} \to 2Fe^{2+}_{(aq)} + 2H_2O_{(l)}$