The key step in the manufacturing of $H_2SO_4$ by contact process is

• A
  absorption of $SO_3$ in $H_2SO_4$ to give oleum
• B
  dilution of oleum with water
• C
  burning of sulphur in air to generate $SO_2$
• D
  catalytic oxidation of $SO_2$ with $O_2$ to give $SO_3$