The diagram illustrates a flow chart for producing fertiliser. In the flow chart, what are W, X, Y and Z?
- AW: $\text{H}_2$, X: $\text{N}_2$, Y: high, Z: $\text{NH}_3$
- BW: $\text{O}_2$, X: $\text{SO}_2$, Y: high, Z: $\text{SO}_3$
- CW: $\text{O}_2$, X: $\text{SO}_2$, Y: low, Z: $\text{SO}_3$
- DW: $\text{N}_2$, X: $\text{H}_2$, Y: high, Z: $\text{NH}_3$