In the Haber process,hydrogen is obtained by reacting methane with steam in the presence of a $NiO$ catalyst. This process is known as steam reforming. Why is it necessary to remove $CO$ when ammonia is obtained by the Haber process?

• A
  $CO$ acts as a catalyst poison for the iron catalyst.
• B
  $CO$ reacts with $N_2$ to form $HCN$.
• C
  $CO$ increases the rate of the reverse reaction.
• D
  $CO$ is highly flammable and poses a safety risk.