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

• A
  $A_{2(g)} + B_{2(g)} \xrightarrow{C_{(g)}} 2 AB_{(g)}$
• B
  $A_{(s)} + B_{(s)} \xrightarrow{C_{(s)}} D_{(s)}$
• C
  $A_{(g)} + B_{(g)} \xrightarrow{C_{(s)}} D_{(g)}$
• D
  $A_{2(g)} + B_{2(g)} \xrightarrow{C_{(g)}} D_{(s)}$