A student investigates the rate of reaction between dilute nitric acid and calcium carbonate.

• The student repeats the experiment using three different particle sizes of calcium carbonate: large chips, medium chips, and a fine powder.
• In each of these three experiments, they use exactly 4 g of calcium carbonate, with the nitric acid in excess.

The graph shows the student's results, along with a fourth trial, P, which was carried out under different conditions: