Carbonates react with dilute acids to form carbon dioxide. A student uses excess carbonate and $100\,\text{cm}^3$ of $0.1\,\text{mol dm}^{-3}$ acid, then records the volume of gas produced at equal time intervals. These results are shown by line X on the graph. The student then repeats the experiment with $50\,\text{cm}^3$ of $0.2\,\text{mol dm}^{-3}$ acid while keeping all other conditions unchanged. Which line represents the results for the second experiment?