Study the dispersion graph below, which shows soil infiltration rates measured at 11 sampling locations in a woodland.
What is the median infiltration rate on this dispersion graph?
21 mm/hour21\text{ mm/hour}21 mm/hour
30 mm/hour30\text{ mm/hour}30 mm/hour
25 mm/hour25\text{ mm/hour}25 mm/hour
31.5 mm/hour31.5\text{ mm/hour}31.5 mm/hour