$A$ $10.0 \, L$ vessel contains $O_2$ at a partial pressure of $600 \, torr$ and $H_2$ at a partial pressure of $400 \, torr$. The gases react according to the following equation upon electric sparking: $H_{2(g)} + \frac{1}{2}O_{2(g)} \to H_2O_{(g)}$. If the reaction goes to completion,which of the following is correct?

• A
  The final partial pressure of $O_2$ will be $100 \, torr$.
• B
  $O_2$ is the limiting reagent.
• C
  The partial pressure of water vapor is $900 \, torr$.
• D
  The total pressure at the end of the reaction will be $800 \, torr$.