In the preparation of sulphuric acid,$V_2O_5$ is used in the reaction,which is:

• A
  $S + O_2 \to SO_2$
• B
  $2SO_2 + O_2 \xrightarrow{V_2O_5} 2SO_3$
• C
  $SO_2 + H_2O \to H_2SO_3$
• D
  $N_2 + 3H_2 \to 2NH_3$