Pupils at Mulima Primary School wanted to find out whether planting grass on a slope would reduce soil erosion. They planted grass on one tray and left the other without. They then set up their experiment as shown in the diagrams below.What error did the pupils make in setting up this experiment?A. Making the slope of the trays different.B. Dripping water from the same height.C. Using only one type of soil in both trays.D. Planting grass on one tray only.