The diagram illustrates three stages in the production of sulfuric acid. sulfur $\xrightarrow{\text{step P}}$ sulfur dioxide $\xrightarrow{\text{step Q}}$ sulfur trioxide $\xrightarrow{\text{step R}}$ sulfuric acid At which stages is a catalyst employed?
- AP, Q and R
- BQ and R only
- CQ only
- DR only