In a typical fuel cell,the reactant $(R)$ and product $(P)$ are

• A
  $R=H_{2(g)}, N_{2(g)}; P=NH_{3(aq)}$
• B
  $R=H_{2(g)}, O_{2(g)}; P=H_{2}O_{2(l)}$
• C
  $R=H_{2(g)}, O_{2(g)}; P=H_{2}O_{(l)}$
• D
  $R=H_{2(g)}, O_{2(g)}, Cl_{2(g)}; P=HClO_{4(aq)}$