Which of the following is an example of a first-order reaction?

• A
  $2 NH_{3(g)} \rightarrow N_{2(g)} + 3 H_{2(g)}$
• B
  $2 H_2 O_{2(aq)} \rightarrow 2 H_2 O_{(l)} + O_{2(g)}$
• C
  $4 PH_{3(g)} \xrightarrow{W} P_{4(g)} + 6 H_{2(g)}$
• D
  $2 N_2 O_{(g)} \xrightarrow{Pt} 2 N_{2(g)} + O_{2(g)}$