The length l (in feet) of a runway is needed for a small airplane to land, given by the equation l = 0.1s² - 3s + 22, where s is the airplane's speed (in feet per second). If a pilot is landing a small airplane on a runway 2000 feet long, what is the maximum speed at which the pilot can land?
a) 20 ft/s
b) 30 ft/s
c) 40 ft/s
d) 50 ft/s