A student places excess magnesium ribbon into $10\text{ cm}^3$ of $0.5\ \text{mol dm}^{-3}$ sulfuric acid. The hydrogen gas is collected, and its volume is recorded at intervals of 10 seconds. The experiment is then carried out again with the same mass of magnesium ribbon, but this time $5\text{ cm}^3$ of $0.5\ \text{mol dm}^{-3}$ sulfuric acid is mixed with $5\text{ cm}^3$ of water. Which graph shows the results of the second experiment?