Which one of the following reactions is an example of heterogeneous catalysis?

• A
  $2 CO_{(g)} + O_{2(g)} \xrightarrow{NO_{(g)}} 2 CO_{2(g)}$
• B
  $2 SO_{2(g)} + O_{2(g)} \xrightarrow{NO_{(g)}} 2 SO_{3(g)}$
• C
  $2 CO_{(g)} + O_{2(g)} \xrightarrow{Pt_{(s)}} 2 CO_{2(g)}$
• D
  $CH_3CHO_{(g)} \xrightarrow{I_{2(g)}} CH_{4(g)} + CO_{(g)}$