The correct statement amongst the following is $:$

• A
  The term 'standard state' implies that the temperature is $0^{\circ} C$
• B
  The standard state of pure gas is the pure gas at a pressure of $1 \ bar$ and temperature $273 \ K$
• C
  $\Delta_{f} H_{298}^{\theta}$ is zero for $O_{(g)}$
• D
  $\Delta_{f} H_{500}^{\theta}$ is zero for $O_{2(g)}$