Two large, plastic tubs were filled with soil. the soil was shaped to create a mound in each tub. the starting height of each mound was recorded. the tubs were set outside, side by side. the lid was left off of tub a and a lid was placed on top of tub b. once a day, one ounce of water was poured over the top of each mound for ten days straight. at the end of the ten days the mound heights were recorded and compared. is this a valid experimental design to test for wind erosion of soil? a) yes, because dirt is very heavy. b) no, because ten days is not long enough. c) no, since the type of soil was not identified. d) yes, steps were outlined for condition with one variable changed, lid on or off.