Darien observed that the water level fell 3.1 millimeters per year over a period of 4.2 years. Tom observed that the water level fell 3.9 millimeters per year over a period of 2.9 years. Dakota observed that the water level fell 4.8 millimeters per year over a period of 2.8 years. Who observed the greatest variation in water levels? Who observed the least variation?