The diagram below 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 of these steps is a catalyst used?
- AP, Q and R
- BQ and R only
- CQ only
- DR only