In the Contact process, which reaction uses a catalyst?
- A$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$
- B$2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$
- C$\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7$
- D$\text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4$