The following reactions take place in the blast furnace during the preparation of impure iron. Identify the reaction pertaining to the formation of the slag.

• A
  $Fe_2O_{3(s)} + 3CO_{(g)} \rightarrow 2Fe_{(l)} + 3CO_{2(g)}$
• B
  $CaCO_{3(s)} \rightarrow CaO_{(s)} + CO_{2(g)}$
• C
  $CaO_{(s)} + SiO_{2(s)} \rightarrow CaSiO_{3(l)}$
• D
  $2C_{(s)} + O_{2(g)} \rightarrow 2CO_{(g)}$