Give the chemical reaction for the preparation of producer gas.

• A
  $C(s) + H_2O(g) \rightarrow CO(g) + H_2(g)$
• B
  $2C(s) + O_2(g) + 4N_2(g) \rightarrow 2CO(g) + 4N_2(g)$
• C
  $C(s) + O_2(g) \rightarrow CO_2(g)$
• D
  $CO(g) + H_2O(g) \rightarrow CO_2(g) + H_2(g)$